Humidity for wood rotting

Question: If I put a piece of wood in a humid environment (basically humid air), it will start to rot. Does the humidity influence the rotting speed, and if so, what is the relation between the rotting speed and humidity?
Answer: Rotting is a biochemical reaction: basically catabolism (breaking down of complex compounds to simpler compounds). Cellulose or lignin or any other structural polysaccharide will not undergo spontaneous hydrolysis. This reaction is catalyzed by enzymes produced by the microorganisms like fungi and bacteria (the term for these decomposers is saprophyte). Saprophytes prefer to grow in humid and warm environments.
So the primary role of humidity is to promote the growth of decomposers. Simultaneously it provides the water required for the hydrolysis.
Answer: I'm afraid this is not a topic of chemistry, the main degradation process in wood decay is a matter of biological attack, of course life needs water so if the ambient is very humid the wood decompose very well if the ambient is dry the wood can be conserved easier.
However wood is not a substance but a complex material, you have to investigate the degradation process for all the components. Chemically holocellulose is the most degradable part of wood, his microcrystalline structure improve the resistance but cycle of imbibitions can blow up the structure leading to a larger surface more suitable for degradation. The hydrolysis of cellulose however doesn't occur at normal temperature only peeling off can occur under 100 °C.
Lignin is more and more durable if a little bunch of micro and macro organism wouldn't decompose it we would be sub-merged by a large quantity of lignin.

Humidity for wood rotting


How do I develop a bass voice?

Question: I am an 'adult male human' and can sing from A2 to G4 comfortably. It would be great if I can sing up to a whole octave below this. I am in no hurry and am willing to expend time and effort. I just want to know what are the techniques and what amount of work would be involved. Also, I would like to know what is the difference between a false bass and a true bass voice and when to employ which.
Comment: A bunch of questions here: Are you sure you have your ranges notated correctly? An adult male who can't comfortably sing below A3 (the A directly below middle C) can't really sing any of the standard male voice parts (at least not as they're normally written). What is your top note? Might you really be a tenor or countertenor instead?
Comment: OK. That's much more believable. On the other hand, you really don't need to go down to A1—there's only a handful of works that call for it! Practically, it sounds like you're a baritone. You shouldn't really need to get much lower than the Eb2—and even then, it's more a "bonus" than a necessity.
Comment: That's actually quite a high range—more or less a tenor range. But the bottom note is quite a lot higher than the bottom note for "typical male voices."
Answer: First off, let's start with the definitions of the terms.
False Bass (or "Fauxbourdon") is a French term relating to the harmonization of plainchant melodies in which the bass would be a 6th below the melody; thus creating a serious of first-inversion chords as the organum would typically be harmonized a perfect-fourth below the melody.
I believe you probably meant to ask about vocal fry, which is like a falsetto, but for the lowest part of the vocal range.
A Bass range is approximately from E2 - E4 though some operatic roles necessitate further range extension.  However, a True Bass range can extend much lower - typically down to Bb1 though many singers - especially Gospel singers and Russians can sing even lower still.
There is a joke in music: "Russia, where the men sing like men...and so do the women!"
As for expanding your range, like anything worth doing, it takes time.  How much time and how much work are up to you - your results will show accordingly.  Practicing too much can overtax your voice and lead to complications.  Practicing too little will not yield desired results.
If you want to expand your range down, you must also expand your range up.  Yes you can absolutely only work on low stuff, but you would only fully develop your low range when you developed your high range as well.
A simple exercise to do would be to start at middle C on the piano (C4) and sing downward chromatically to your lowest note.  It should be relaxed; not forced.  Try 3 times to sing the note clearly.  If it doesn't happen, leave it be and try again the next day.  The same should be done for singing upwards as well.  The idea is that over time you will eventually develop the necessary flexibility to sing those pitches; thus expanding your range.
Another simple exercise for developing a purer tone and increasing flexibility would be to sing "Nae-Yah" (spelled phonetically) outlining a perfect-fifth interval with each syllable.  "Yah" is held as you outline the descending triad and the final 5-note scale that outlines the original interval; which can either be slurred or articulated with "ha".
There are a plethora of vocal techniques to list that can help you, but most of them should be demonstrated in some way, and you would not otherwise glean any assistance from my answer.  My recommendation would be to find a friend who is a trained vocalist and have them show you some warm-up techniques that will "open" up your voice and give you more flexibility.
Two more thoughts:
1.) Always warm up your voice by singing down before you sing higher (in your case make sure you sing higher as well!)
2.) Every voice is unique, and an adult man with a high voice is actually a very attractive sound.  So much so that in Italy they use to castrate boys before they reached puberty so that their voices would not drop and they retained the boyish purity of tone with the presence of a man.  Having heard a recording of one of the last male castrati I can say that it is quite a unique and mysterious sound.
At any rate, I hope those thoughts were helpful and answered your questions.
Comment: Thanks for the detailed answer, I messed up my range in the question, corrected now. Sorry about that.
Comment: Thanks for the update - that makes more sense.  Regardless of where your range lies, those techniques should still be able to help you!
Answer: I've found this to be quite a challenge personally. Developing the bass tone is difficult precisely because focusing attention tends to induce muscular tension and raises muscle tone making lower vibrations more and more difficult to achieve.
If at all possible, do your practice first thing in the morning and stop when you notice tension rising.
Bass note require more breath support. Try to summon the note from below with a lot of breath. Tension is applied only to select the pitch; don't ever push lower by squeezing anything.
Comment: As a singer with a A1-A4 range, I actually find the low bass notes to use less air than the rest of my range.  The resonance is also much lower.  It isn't in my mouth, it's in my throat.  Usually if I stick my chin out a tiny bit the volume output can be higher.
Answer: I'm 14 and my my range is A1-F4.
I used to have a high voice before I hit puberty and last summer it dropped to a D2.
I'm in choir and the lowest singer in Highschool. I have worked on my voice for over a year and have extended my range to an A1-F4.
In the beginning I just kept practicing singing low and I didn't have much improvement, so I started working on singing higher because
I could only hit an A4, but when I extended it I noticed that my voice was getting lower. Since then I've kept working on my upper range and occasionally working on singing lower, and in a years time my voice has a range of A1-F4
Comment: How's the progress with your technique?
Answer: Being a bass is not by working only, you have to have it basically... and with a range like that, I wouldn't get my hopes to high, but it can be achieved with techniques I guess! I'm 17 and my range is Eb2 - G4 my vocal coach told me I could grow up to be a bass... but with A2 as the lowest note, i'm not so sure about that...

How do I develop a bass voice?


Raspberry B+ remove USB ports

Question: Can anyone please tell me if the USB physical ports can be removed from the Raspberry Pi B+ ? I am using it in an audio app so I don't want to see them on the case.
Answer: Of course you can. Go ahead and desolder them. It should't be a problem.
Take a look at this: http://freneticrapport.blogspot.com.au/2013/01/first-raspberry-pi-model-512mb-off-re_24.html
Answer: Both the two double-port USB jacks and the RJ45 ethernet jack can be desoldered from the board - given careful handling and removing short circuiting solder. I mention the ethernet jack, as it is a big and ugly as the USB and might also show up on the case. If ethernet is needed and the jack needs to remain on board, however I fail to see the benefit of removing USB. I'd rather look for a case that just hides their existance.

Raspberry B+ remove USB ports


Processing photos with Lightroom 4 and Nik Software

Question: My understanding is that when editing a photo with Nik Software, you cannot process a raw file directly. Which format would be best to use amongst the option the plugin provides? Also, if processing raw files directly is not possible, doesn't that make shooting raw a bit less useful when using this plugin?
Answer: TIFF is a good lossless format to use when raw is unavailable.
It is still a good idea to shoot raw because it gives you ultimate flexibility. You still have to process the raw file before you export to tiff, so you have another 'layer' of adjustment to refine your image with before you put it through the Nik software.
It also means you are not losing any quality by using JPEG compression.
And of course, you might want to edit the file later without the Nik software, so it's good to have it in raw anyway.
Comment: with TIF though, I get an image five times as bigger. is it normal?
Comment: Five times bigger than what?
Answer: Shooting raw is still beneficial because you have more latitude in making initial adjustments like exposure and white balance.  You'll want to do as much in Lightroom as you can before exporting to Nik.  The most common format to use is TIF, as it is lossless.
If you have Photoshop CS, your best bet would be to export to Photoshop, and then do your Nik adjustments in layers (or better, with smart objects), giving you more control to tweak each layer.


Quickreport - pagenumber does not increase when page changed

Question: I am using the OnNeedData feature with an array feeding a number of qrLabels in a detail band configuration - (Delphi 2010 and QuickRep5) - On a multi-page report, I need to detect every next page (2, 3, 4 etc) and modify the first line in the detail band.
Everything works fine except that the internal page number variable (QuickReport.QRprinter.pagenumber) does not increment before the first line is printed (or shown).
As an exemple, if I print a spam removed of labels with the caption equal to the page number,
the first line of page 2 will show Page 1 and the rest of the lines on that page will show correctly Page 2 same thing happens for all other pages.
Can someone help me to find a way to properly identify the beginning of a new page, I have tried OnEndPage, OnStartPage, BeforePrint, etc. without success - the counter always increment after the first line of the detail band has been printed...
Thank you
Answer:
Make sure that the Quickrep1 has a PageheaderBand
Then examine the QuickRep1.QRPrinter.PageNumber in the OnBeforePrint event handler of that PageHeaderBand.


In British heraldry, what happens when a woman with a coat of arms marries someone without?

Question: I'm aware this isn't very time-specific, and not sure if the right SE, but what happens when a woman who belongs to a noble family and has a coat of arms, marries someone without a coat of arms etc.?
Comment: Leo, the College of Arms website discusses the descent of arms here: http://www.college-of-arms.gov.uk/resources/the-law-of-arms

It describes heraldic heiresses, etc.

Incidentally, you can be armigerous without having a title. Note the info at http://www.college-of-arms.gov.uk/services/granting-arms -- you don't need a title, but you do need some money!

Of course, this is all modern-era.
Comment: There were (are) quite a few heraldic heiresses. Heraldry started as military identification, but it didn't take long to become an identification of lineage instead. The military use is why women weren't supposed to display arms on a shield, though. (See the quotes I posted in my answer for some comments about that.)
Comment: Suppose she can bear arms for whatever reason. (I didn't know the definition of armigerous, thank you for bringing that up.) So you think that if she married, she would lose that right?
Comment: Yes, but I'm afraid I have more questions (my knowledge of heraldry is largely confined to Game of Thrones.) Do titles and arms go hand in hand, as in one who has a title is armigerous? Also, I'm not worried if it's not wiki-quality information, so long as it's a vaguely coherent answer and you're decently reputable I'm satisfied. The question came from no more than curiosity.
Comment: Okay, thank you, that's essentially everything I want to know. I'll accept it if you put it into an answer.
Comment: Nothing happens. Titles are not conferred or changed by marriage in normal English practice. Also, coats of arms are for military unit shields and banners, so women didn't have normally them.
Answer: Modern practice is this, according to An Heraldic Alphabet (p. 231, 1996 edition) by former Clarenceux King of Arms, J.P. Brooke-Little:
(edited to add -- this was a new addition to the 1996 edition, mentioned as a change in the heraldic laws.)

"...If a woman entitled to arms marries a man who does not have arms,
  she may continue to use her maiden arms, but on a shield not a
  lozenge and on a banner, but differenced by a small escutcheon of
  suitable tincture displayed either in the canton, centre chief point,
  or in such other part of the shield as marks of difference are
  customarily displayed."

A lozenge is a diamond shape, traditionally used for women's arms in some cases instead of the shield. An escutcheon is the shield shape. So "differenced by a small escutcheon of suitable tincture" means that she would display the arms with a small shield shape of a suitable color added, in the upper corner, the upper center, or elsewhere.
In 1903 the topic was discussed in letters to the publication Notes and Queries. This is long, but it's a fascinating look at some slightly less modern opinions:

"Mr. Udal suggests that an armigerous woman who marries a non
  armigerous man may still display her own arms. But how? Her husband
  has no shield, so where are the wife's arms to go? She cannot bear
  them apart, on a man's shield, because she is not a man; she cannot
  use a lozenge, because she is neither maid nor widow. Of course if the
  non-armigerous husband dies, and the armigerous lady marries an
  armigerous male, then her arms can be impaled with her second
  husband's, or put in pretence, if she has no brothers.
"Not long ago a friend of mine consulted me as regards a friend of
  his, a married woman, who wanted an armorial book-plate of her own,
  apart from her husband's. This seems to me, as I told him, impossible.
  No married woman can hear her arms apart from her husband's shield,
  whether impaled or in pretence. The only (apparent) exception is when
  a peeress in her own right marries a commoner, and her shield, with
  coronet and supporters, is placed side by side with his. But even then
  her arms, ensigned with the coronet of her rank, are also placed in
  pretence on the husband's coat.
"The armigerous woman marrying the non-armigerous man does not forfeit
  her arms. They are there, ready for use when they can be used. As long
  as her husband is non-armigerous they cannot be used--cannot be in
  evidence. If the husband becomes armigerous, or she marries an
  armigerous man, then the impediment is removed." -- March 7, 1903

"...There can be no consistent reason why a lady of coat armour in
  these more enlightened days (when a married woman is considered a
  separate entity, and is allowed to hold property independentiy of her
  husband) should not bear her paternal arms on the feminine lozenge if
  she pleases, and that whether her husband be armigerous or not."
"It would seem that a lady married to a non armigerous husband should
  bear her arms on a lozenge, after the analogy of a peeress in her
  own right who is married." -- April 18, 1903

A bit more about the modern Law of Arms in England and Wales (Scotland has its own separate heraldic authority) is on the College of Arms website.
Another thing that might be relevant -- the College of Arms recently posted a ruling on the arms of individuals in same-sex marriages. It's relevant to this question because of this section:

"(3) A married man will continue to have the option of bearing his own
  arms alone. A ruling of the Kings of Arms made on 6 November 1997
  allows a married woman to bear her own arms alone differenced by a
  small escutcheon. That will continue to be the case but the addition
  of the mark of difference will forthwith be optional."

So the tiny escutcheon is now optional, I guess.
Edit:
This is only peripheral to the OP's question since it doesn't address the non-armigerous husband issue, but there is an article, "Heraldry and the Medieval Gentlewoman", in the March 2003 issue of History Today. It does discuss some interesting aspects of heraldry in the female line and might be of interest. Unfortunately, the article is behind a paywall, but you may be able to read it online through your local library.
The article's author, Maurice Keen, quotes a fifteenth-century lawyer saying something very similar to the words of the March, 1903 letter writer above:

"In cause arms were first ordained that a mighty noble warrior might
  be known from others … if arms be given to a man and he hath issue a
  daughter, to what intent should she bear his arms with a difference?
  To none; because she shall never wear coat armour nor come to wage war
  in the field … Wherefore understand that she beareth arms only to be
  known of that noble blood and [for] the continuance of the right to
  those arms in them that shall come of her."
Comment: Thanks! I wish I could have found more that specifically discussed earlier periods. Given more time I probably could, so if I find anything I'll add it.

I love this question because I had never actually thought about this particular aspect of heraldry, so I got to learn something.
Comment: Superior answer.  OP should accept this. Far better researched than mine; based on your prior publications, I should have known that you'd be able to supply a better answer.
Comment: Your comment highlights one of the things I love about stack exchange. Most answerers I've encountered seem to genuinely care about improving the community's collective knowledge and providing a reliable resource to future readers, rather than focusing on bolstering their status and farming reputation.
Comment: Heh, thank the friend of mine who brought it up on a whim during a Computing lesson. (Obviously we were making productive use of our time.)
Answer: I'm sorry, but Mark's answer strikes me as almost entirely wrong. He says that nobles cannot have professions etc - I don't believe that to have ever been the case in England, though it was in some continental countries. He also says that the father "gives" his lands and titles to his eldest son, younger sons join the army or the clergy - well not necessarily!
Generally, nobles had little or no freedom as to whom inherited the patrimony and certainly none over who inherited the title/s - that was set by the terms on which the title was bestowed, and almost always "in tail male" - that is, the eldest son got the title and most of the land, with some provision for daughters' dowries. Whislst a father could choose to grant unentailed lands to a daughter, he most certainly could not grant her any of his titles. Even now in England there are very few titles above baron that can be inherited by a woman - a fact many male peers feel unfair and outmoded.
On the position of an armigerous woman marrying a non-armigerous man, I have to confess I don't know, and my book on heraldry does not touch on it. However, it was common in the mediaeval period for men to claim titles "by right of his wife" if she had no brothers to inherit; John of Gaunt became Duke of Lancaster by marrying Blanche of Lancaster.
I think there is some confusion here between bearing a title - Baron or above - and having a coat of arms - not every armigerous family is or was noble. Also, the position of an heraldic heiress is slightly  different, referring to all the daughters of an armigerous man, not necessarily noble, who have no brothers. In this case the wife's arms are displayed on her husbands as an escutcheon of pretence - a small shield in the centre of the husband's arms - as he is "pretending" to be the head of his wife's family. Any children - male and female - would then quarter their arms with both parents' as opposed to just their father's.
Answer: In November 1997 the three kings of arms of HM College of Arms in London decided that a married woman whose husband was not armigerous could display her father's shield, differenced by an aptly placed and aptly tinctured inescutcheon. In 2014 they declared that the inescutcheon was now optional. The arrangement for married women is as above but with the difference provided by a lozenge.
Answer: I corresponded with the Chief Herald of Ireland's office on this question some years ago. My question related to arms inherited by an heraldic heiress, who herself had only heraldic heiresses (pre-Republic of Ireland), with that latter heiress marrying a non-armiger, and having male heirs. The Chief Herald's office said that such heirs possessed the abeyant right to quarter the coat of arms in question, should they petition for a grant of arms themselves.
Answer: The answer depends on the country and the era.  You've limited the question to England, and implicitly to the pre-modern period.  The best resource is probably DeBrett's, but it isn't terse, it isn't easy to use, and it isn't free.   I'm going to summarize with broad generalizations.
Titled nobles are armigerous (have the right to arms). A given Noble may have multiple titles (he may be the Earl of X and the Marquis of Y, and the W of Z).  If you don't have a title, you are just a Lady or a Gentleman.
A father needs to ensure that his children have a living or an income. Since nobles cannot have a profession or engage in trade, that living is one of four forms (1) land & title, (2) military (3) clergy, (4) marriage. Fathers have a limited number of titles and lands; the normal option is to give the bulk of the titles & land to the eldest son. The second son joins the military, and the third & successive sons join the clergy. (Later in history the father would need to settle some lands on the second son to ensure that his military career would prosper as an officer, but that's outside the scope of OP's question).
Daughters cannot join the clergy or the military, so they are limited to either marriage or titles. Every title granted to a daughter diminishes the patrimony (and power, and wealth) of the eldest son.  The eldest son also has a customary responsibility to advance and protect the welfare of his siblings, so it is a bad idea to diminish their welfare.
So to wrap up, although a father may choose to grant a title & lands to a daughter, that is usually done as part of a dowry to make a better marriage.  HOWEVER, if the daughter is the only child, then the titles may devolve to her (and then convey to her husband) at various points the crown may need to approve the conveyance.  In general Scottish titles are easier for a woman to inherit. Here is a list of titles that convey in the female line
OP asks what happens if an armigerous woman marries a man without arms.  The answer is probably going to depend on politics.  If the man is common, then her titles are probably forfeit -  because a baseborn man has no chance of protecting the titles or serving the king (he is almost certain to have no training in arms, and nobody will follow him in war).
If the man is noble, then her titles probably convey to him and to their children, but that will only happen if he can defend the title (again this part depends on which era in history. If we're talking Game of Thrones/War of the Roses era, then EVERY noble must be prepared at all times to defend his lands and titles.  Kings permit their nobles to carry out limited warfare among themselves because it ensures that he will be supported by the strongest nobles in the event of a national war. (I suppose one could consider the king to be pursuing an optimal portfolio strategy of military strength).
Given the breadth of the question, this is a collection of unsupported generalizations.  If I could narrow it down, and if I had lots of time, I could find specific references & examples.  If you want to read further, try the wars of the roses (which George Martin admits is one of his sources for Game of Thrones).  Alternatively look at the history of Empress Matilda, which is brilliantly, if fictionally covered in the Brother Cadfael novels.
It is also worth consulting She-Wolves (As an American, I haven't seen this yet. Waiting on BBC America)  which is based on a book  (I have a copy in the "to read" pile, but based on an interview with the author, I think it will go a distance to revising our understanding of women in English history.)
Comment: @MarkCWallace Sorry, but this is just wrong. A peer could **not** grant titles to his daughters, and would probably be reluctant to grant lands, (even assuming they weren't entailed on the eldest son) which would break up the estate. And you are confusing armigerous with the peerage - it was - and is - entirely possible to have a coat of arms **without** being a peer.
Comment: As I've said before, it's only a mild curiosity - I have no knowledge or great calling to explore heritage and heraldry. What you and litlnemo have told me was everything I needed to know and more :). I appreciate all the effort!


Curve from a Douglas-Peucker line

Question: When generating a line using the Douglas-Peucker Algorithm the resulting object is a line with steep angles, I would like to "smooth" that line and making it look less "squarish" using curves, like bezier curves: there is an algorithm for that ?
Answer: take a look at: http://ricegisinfo.blogspot.co.at/2009/12/tuesday-tools-mapshaper.html
the  visvalingam-algorith is described here:
http://www2.dcs.hull.ac.uk/CISRG/publications/DPs/DP10/DP10.html
mapshaper (http://mapshaper.com/test/MapShaper.swf) an online flash-tool,  uses a special version of  visvalingam, which should do what you want, but you will need to contact the author for more infos about this special version of visvalingam . the author is matthew bloch (http://maps.grammata.com/)

Curve from a Douglas-Peucker line


How do I prove by induction that, for $n≥1, \sum_{r=1}^n \frac{1}{r(r+1)}=\frac{n}{n+1}$?

Question: Hi can you help me solve this:
I have proved that $p(1)$ is true and am now assuming that $p(k)$ is true. I just don't know how to show $p(k+1)$ for both sides?
Comment: Do you mean $\sum_{r=1}^n \frac{1}{r(r+1)}=\frac{n}{n+1}$?
Comment: Yes! sorry I do not know how to format like that.
Comment: You can learn some [here](http://meta.math.stackexchange.com/questions/5020/mathjax-basic-tutorial-and-quick-reference) and at some of the references that you'll find there.
Comment: Thanks I have just learnt it! :)
Answer: Hint: $$\sum_{r=1}^{k+1}\frac{1}{r(r+1)}=\sum_{r=1}^{k}\frac{1}{r(r+1)}+\frac{1}{(k+1)((k+1)+1)}.$$ Now use the inductive hypothesis and see if that gets you anywhere.
Answer: If you want to do it by a conventional "blind" induction, suppose that for a certain $k$ we have
$$\sum_1^k \frac{1}{r(r+1)}=\frac{k}{k+1}.\tag{$1$}$$
We want to prove that
$$\sum_1^{k+1} \frac{1}{r(r+1)}=\frac{k+1}{k+2}.\tag{$2$}$$
Note that the left-hand side of $(2)$ is the left-hand side of $(1)$, plus $\dfrac{1}{(k+1)(k+2)}$.
So we want to prove that
$$\frac{k}{k+1}+\frac{1}{(k+1)(k+2)}=\frac{k+1}{k+2}.\tag{$3$}$$
It seems reasonable to manipulate the left-hand side of $(3)$ and see whether we get the right-hand side. A common manipulation is to bring the expression to the common denominator $(k+1)(k+2)$. We get
$$\frac{k(k+2)}{(k+1)(k+2)}+\frac{1}{(k+1)(k+2)}.$$
This is equal to $\dfrac{k^2+2k+1}{(k+1)(k+2)}$.
But the numerator is equal to $(k+1)^2$. Cancel a $k+1$.
Remark: The algebra at the end is neater, and closer to the informal "telescoping" argument, if we observe that $\dfrac{1}{(k+1)(k+2)}=\dfrac{1}{k+1}-\dfrac{1}{k+2}$. Thus
$$\frac{k}{k+1}+\frac{1}{(k+1)(k+2)}=\frac{k}{k+1}+ \frac{1}{k+1}-\frac{1}{k+2}=1-\frac{1}{k+2}=\frac{k+1}{k+2}.$$
Answer: $$\sum_{r=1}^{n}\frac{1}{r(r+1)}=\frac{n}{n+1}$$
for $n=1$ we have $\frac{1}{1(1+1)}=\frac{1}{1+1}$ suppose that
$$\sum_{r=1}^{k}\frac{1}{r(r+1)}=\frac{k}{k+1}$$ then
$$\sum_{r=1}^{k+1}\frac{1}{r(r+1)}=\sum_{r=1}^{k}\frac{1}{r(r+1)}+\frac{1}{(k+1)(k+2)}=$$
$$=\frac{k}{k+1}+\frac{1}{(k+1)(k+2)}=\frac{k(k+2)+1}{(k+1)(k+2)}=$$
$$=\frac{k^2+2k+1}{(k+1)(k+2)}=\frac{(k+1)^2}{(k+1)(k+2)}=\frac{k+1}{k+2}=\frac{(k+1)}{(k+1)+1}$$
Answer: HINT:
$$\frac1{r(r+1)}=\frac1r-\frac1{r+1}$$
and
$$\frac{n}{n+1}=1-\frac1{n+1}\;.$$
Comment: I suggest that the downvoter think a bit more about how these two observations can be incorporated into a proof by induction. (Indeed, one has only to look at André's remark.)


My dryer vent was setup to go thru the wall, up 2 stories, to my roof

Question: My dryer vent was setup to go thru the wall, up 2 stories, to my roof.....
I have a major problem with this type of setup, as every couple of months, my dryer starts taking forever to dry. When I open the dryer door, I see lots of moisture on it. We then have to take off the duct from the wall connection and we find lots of wet lint that needs to be taken out. We run a blower thru the vent, from the bottom, hoping it will dry/clear out whatever is in the vent between the bottom floor and the roof. I'm thinking that with heavy rain, water must be getting in somehow from the roof vent. I can only imagine what's in there... I don't have access to the roof, as it is 2 stories up (I live in a townhouse). My question here is.. does anyone have any suggestions on how to fix this problem? I have lived here over 20 years and am so fed up with this... I'm getting ready to just leave the duct behind the dryer with a panty hose on it, but have read this is not recommended. I live in Florida and already have lots of humidity.
I appreciate any suggestions!
Comment: If a service from your townhouse runs beyond the bounds of your townhouse, there should be some sort of owners association or site maintenance that you can work through to get the part beyond "your" section of building serviced. i.e. if you pay any "maintenance fee" where does that go? Who would fix the roof if it leaked? Follow the money.
Comment: Thanks for responding!The roof has huge tiles, which I'm told, easily crack if stepped on the wrong way. I wouldn't dare to even try walking on them, even if I could get up there. The roof, unfortunately, is now considered the home owner's responsibility.
Comment: The dryer at our house runs multiple times a day.  I have kids playing sports, we do outdoor stuff, we dry blankets all the time and I never see any lint in our dryer venting.  We also don't have the best dryer in the world but it does have a lint trap that we keep cleaned regularly.  If you are having issues with lint in your venting you need to start with why isn't your dryer stopping the lint from hitting the vent.  I just did my bi-yearly vent cleaning... I got two cotton ball sized pieces.
Comment: Wow! What model dryer do you have? I do at least 2 loads per day, and clean out the lint trap before turning the dryer on each time. My dryer is a Kenmore, only 2 years old. The heating element failed on it this past month, and when I opened it up to replace the part, there was a carpet of lint over EVERYTHING inside. The lint on the bottom was even stuck to the metal, like it had been wet and dried there - making me wonder if the water from the duct was going into the dryer itself... What a mess!
Answer: As a general rule, dryer vents should be cleaned out with a brush (attached to a long chain or rope or flexible rod so they can be run through the duct) every 6 months or so. If the arrangement of the vents in your townhouse community is such that you cannot easily do this yourself, you should take it up with whatever owners association or management company there is overseeing the site, since lint clogged dryer vents can be a source of fires, which should be a concern far beyond the impact on your dryer's performance.
If you applied something like a leaf blower to the bottom of the duct, you may have blown the cap on the top right off it, which would add to your problems.
Comment: Thanks for responding! Yes, we use something like a leaf blower to blow, from the bottom up. I'm not even sure it helps any, to be honest. But when we undo the silver duct from the wall connection, there is lots of wet lint there. Clearing this out and blowing for a few hours gets the dryer working properly again, at least for another couple of months. I'm just tired of having to keep doing this, and am considering leaving the duct off and just covered with a panty hose.
Answer: Since my dryer vent was very long too, I bought this kit from Amazon, along with extra rods.

Please note the following:

Use the set screw to attach the brush to the first rod.
Use a pair of pliers to tighten the connection and duct tape to hold the joints in the rods in place
Turn on the dryer in air-dry mode to get some air flow going.
Remove the louvers carefully from the vent cover.
Rotate clockwise only, otherwise the rods will loosen.  Add each rod to the system gradually.

I ended up using 7 rods for a total length of 21 feet.  I got so much lint out of the vent that it could fill up two buckets.
Comment: This would be if you can access the outside end of the vent, (since you mention turning the dryer on to blow air) correct?
Comment: Ecnerwal: yes, but you can also go the other way by disconnecting the dryer from the vent.  You can read the user manual at http://lib.store.yahoo.net/lib/rewilliams/linteaterusermanual.pdf .
Comment: Thank you for this suggestion! The outside end of the vent is on the roof, 2 stories up - not accessible for me at all. Would need to go from bottom up and I don't know how many turns there are from the bottom floor to the roof vent.
Comment: ReallyFedUp: I think using a circular brush to get the lint out should work much better than using a blower, as the lint sticks very well to the vent and is often compacted.  Assuming laminar flow, the air velocity at the inner surface of the vent is almost zero.
Comment: ReallyFedUp: Since the kit is inexpensive, you can always see if using the brush with 4 rods (for a total length of 12 feet) will make a difference.


iPhone no longer syncs automatically when connected to PC

Question: I use my iPhone for work. Our IT technician formatted my hard drive and re-installed the operating system and now my iPhone no longer syncs automatically.  Any suggestions?
Answer: I'm guessing (since you do not state this in your question) that iTunes tells you something along the lines of "This iPhone has not been synced with this computer before so now I have to erase everything on your phone." This is because after the reformat, your iPhone and computer no longer recognise each other because the computer is seen as a different computer.
If this is the case, I don't think you have any choice but to setup your phone anew. Fortunately, you can at least transfer your iPhone content to your computer before you do this by following the instructions at iTunes Store: Transferring purchases from your iOS device or iPod to a computer and/or Downloading past purchases from the App Store, iBookstore, and iTunes Store. I hope this answers your question, but like I implied earlier: some more information would be nice :-)


Employer's website is "web scraping" from another site, can I be liable even though I did not write that code?

Question: So recently I came about an article about web scraping, and it said how most of the time scraping is illegal.  I did not know this in past, and did not know the website I was working on  was using a script to gather information from another website. Of course I don't work for this business anymore but I did let my boss know. My concern is, what if my company gets caught, am I liable?  I am scared since I did use the file that had the actual code-block that did the web scraping.  I did not write this code, but I did use the results that it extracted.  However, at the time I did not know this was bad, or knew even what the process was of getting the information that I was using.  Should I be concerned?
Comment: This question is a better match for [law.se] (more expertise over there I guess)
Comment: web scraping is often the last resort technics used when the data are not available in a usable format and the source is not willing to make it available. I think the discussion belong here.
Answer: Firstly, the information provided in my answers is for general information purposes only and is not intended to be legal advice for individual situations and should not be relied on in taking legal action of any kind. Merely reading my answers does not create an attorney-client relationship. If you believe you may have a claim or defense based on something you read in my answers, you should seek independent legal advice from the lawyer of your choice.
The case of copyright infringement can be serious. An infringer is ALWAYS liable to the copyright owner for the infringement (it is a personal tort action). There is a separate question of whether the infringer's employer will step in and defend and indemnify the employee for the employee's infringement.
So, the employee could be civilly liable, but she or he could face criminal liability as well for knowingly infringing a copyright. The supervisor telling the employee to do it could create criminal and civil liability for the employer in addition to, not in place of, the liability of the employee.
All of this does not answer whether any copyright holder is likely to pursue their rights.
Comment: You forgot the part where I did not know the company had a script that was violating the terms of another website at the time I was working there.
Comment: no, i didn't forget that. You either were infringing the copyright through your actions or you were not. If your company was doing bad things and you never used the data in any way, you were not infringing the copyright of the website. That is a separate question. I read your OP as indicating that you had personally used the data. Ignorance is not a defense.
Comment: Doesn't this overlook that the *company* is liable, not the individual employees (especially since the employee did not initiate the behavior/write the software)? This may depend on country.
Comment: It does not overlook that the company is also liable.
Comment: Im pretty sure our data entry employee will not get in trouble just because he worked with products who had prices that where derived through some value which was illegally obtained....or will he?  Seems like we could all be be breaking the law now, any software we use could have illegal ties without us knowing.
Comment: That is not an equivalent scenario.
Answer: I would have to disagree with the other answer. It has got to do with a lot more than that. Did your employer at the time know his website was scraping? If so, you are in no trouble at all. Writing code that scrapes a website is not illegal, executing them is. If your employer knew this is how his website was programmed, and he agreed with it, he/the company is responsible upon hosting the script and executing the script. Because it's (most likely) prohibited in the TOS of the scraped site. However I do think that if your employer did not know the website was functioning because of the scraping script, the story might be slightly different. But even then, you did not write that code, had no knowledge that the code existed and it was not your responsibility.
Comment: My boss had no idea either, he doesn't know what any web development stuff is, let alone how web scraping works.  He knew prices where being draw from another site, but just like me never thought it would be illegal.  I always thought that they where paying for a service to the that info, or the website they where getting it from had no restrictions. I just figured about this whole web scraping stuff after reading an article about some companies going to court over it.  I told one of the coworkers there about it, and I am going to tell my boss too.
Comment: Still, don't think you are in any trouble. In the past this would have been more serious, nowadays there are better rules in place for violating website's TOS (to protect the users). It won't count as CFAA as fast as in the past, unless you are using special techniques like deceive IP-blocks etc. Probably if the website being scraped notices it, they probably send a cease and decist letter before taking legal steps. I'm talking about scraping non-copyrighted work (facts and numbers are not copyrighted) and I'm not 100% sure about it, but I read quite a few things about it.


Use of original languages

Question: How should we view the use of the original Biblical languages (Hebrew, Aramaic, Greek) in questions and answers? Should transliteration be encouraged for the sake of those who are less familiar with the languages or lack proper font support to view them? Should the original languages be encouraged whenever possible and responses edited to replace transliterations? Or should it be up to whatever the poster is most comfortable doing?
Answer: This site is supposed to be a site of experts in Biblical Hermeneutics (or perhaps exegesis), and as such, one would expect that community to be able to handle a few Hebrew or Greek characters. To push it further, can I trust an answer from someone claiming to know what a Greek word means, but can't understand the Greek characters?
I often see people regurgitating answers about the meaning of a word just because they heard someone else say that. This site does not need that. I want someone to answer my question who knows what he is talking about.
Comment: +1 in that we should be able to handle a few Hebrew and Greek characters (heck, even *Google* can handle it)
Comment: Well, yes and no.  If the "regurgitated" answer really does answer the question, then I see no harm in it.  That seems to be something that occurs SE-network wide.
Answer: The author should feel free to include whatever information best suites their question.
There may be subtleties of meaning in the original texts that can be lost when translated to English. Where precision is paramount, using Aramaic, Hebrew, or Greek text may be most appropriate.
But going to the other extreme will only exclude people from understanding and learning. There's no reason to go overboard with biblical languages when the English words work just as well.
There shouldn't be any particular "policy" that that original biblical text must (or must not) be in their original language. Use whatever makes the question as clear and descriptive as possible.
Answer: A good, free, online resource for coping with Hebrew and Greek is the Transliterate.com service provided by Logos Bible Software.
If I copy/paste "πῶς οὐχὶ καὶ σὺν αὐτῷ τὰ πάντα ἡμῖν χαρίσεται;", for example,  and click the "transform" button, it spits out:

Latin transliteration = pō̂s ouchì kaì sỳn autō̂i tà pánta hēmîn charísetai?
Society of Biblical Literature = pōs ouchi kai syn autō ta panta hēmin charisetai?

Or for Hebrew, יְהוָה רֹעִי, לֹא אֶחְסָר will produce:

Latin transliteration = yhwh ro'i lo echsar
Society of Biblical Literature = yhwh rōʿî lōʾ ʾeḥsār
BETA Code = Y:HWFH RO(IY, LO) )EX:SFR

This could be helpful for BH.SE in two ways:

For those providing answers, it's a quick way to get the transliteration of the original text for the aid of reader who need it.
For readers who lack the languages, and run across untranslitered terms in an answer of interest, they can do the copy/paste > transform operation themselves.

I wonder if it has an "API"?
Comment: Something tells me that there is no *free* API because it's run by Logos.
Comment: PS swasheck and I's research interests lie in making the powerful original language search capabilities of software like Logos, Bible Works, Accordance, etc. available for free
Comment: @Daи - would love to see that. I've used some of the [Crosswire](http://www.crosswire.org/index.jsp)/[Sword Project](http://www.crosswire.org/sword/index.jsp) apps, but I've been using Bible Works since early/mid 90s, and would now find it difficult to do without, although I do keep trying apps from Crosswire, etc., in the hope that they might get close. At least BWks has a reasonable (for me) pricing policy - don't get me started on Accordance, and as for Logos......
Comment: I feel ya on the price point. I now have more invested in Logos than in one of my previous vehicles. I ended up going with it because of its longevity (Libronix --> now), because buying digital works is an investment I want to be able to continue having access to for the rest of my life. And because few other tools have as many resources that are non-Protestant (commentaries and devotional resources).
Answer: The answer should fit the question.
If the question is appropriate here but the OP appears to be more of a novice, then the answer should include transliteration along with the original languages.  Otherwise, the OP will not gain as much benefit as they might have hoped for.
On the other hand, if the question includes the original languages you can assume the OP is more of an expert.  In that case, transliteration is not as necessary.
The bottom line is that I think it is always OK to include the original languages in the answer.  If it looks like that might be over the head of the OP, then include transliterations as a courtesy.
Answer: I think it should be fine to put the Hebrew or Greek in there, though you should usually have the transliteration also.  The real question might be is if SE has an easy way to insert that text without monkeying with the HTML or inserting an image.
Comment: No need for HTML or images, the text can be entered directly (copy and paste some Greek from another site to see).

Use of original languages


Staying out late to study in the library

Question: I am a Muslim girl in college, and I end classes at 5:00 PM. I was thinking that I would stay on the school's library till 8:00 PM to study. Of course, by this time it will be dark outside and I will travel back home by myself with public transportation. It takes me about an hour to get back home. Is this permissible, or should I go home and study?
Comment: Why do you think this would be permissible or impermissible?
Comment: Are you safe? I would be concerned about your safety in some cities.
Answer: There are Hadith saying that a woman cannot make a journey alone "of a night", but this refers to the length of the journey rather than the time of day itself. This page has the sources and more explanation.
There are also Hadith which say that children should not go out at night because of Jinn:

Cover your dishes, close your containers, shut your doors, and keep your children indoors at nightfall, for the Jinn are out and about and stalking. - [Sahîh al-Bukhârî]

However, this only applies to children of course.
There doesn't seem to be any Hadith, and certainly nothing in the Qur'an, which forbids woman being out in the dark, unless it is making a long journey. So I think therefore it is a case of everything being Halal unless specifically made Haram...
You should of course consider your own safety travelling alone at night, but there doesn't seem to be anything haram about it.
Answer: This depends on the condition of your travel/route. If the route you travel has people around during your journey (i.e. no risk of being harassed or assaulted), then I believe it is not Haram to do so.
Of course it is always better and you're encouraged to study at your home after it gets dark. This is due to the fact that women are in much higher risk of assaults at the times of darkness.


How does happiness work?

Question: In the tab Small Village in the Kittens Game, there is an option to send hunters (when there is enough catpower available) with a happiness percentage in front of it. How is this happiness calculated or influenced by me, and what effect does it have on anything (probably the resource gathering from the send hunters option?)?
Answer: Happiness goes down by 2% for every kitten you have after the fifth kitten (so the sixth kitten will drop it by 2% from 100% to 98%)
Happiness is the efficiency of your kittens. For example, a farmer (without any upgrades) normally will yield exactly 1 catnip per tick, but at 98% happiness they'll only yield 98% of that (0.98 catnip per tick)
Hunters can bring in items from the forest (I've only seen fur and ivory), each will increase the happiness by exactly 10%, cumulative (getting both increases by 20%). The amount of each of them only affects how long the bonus lasts, not how many the happiness bonus is
EDIT:
Also, Amphitheaters reduces unhappiness by 4.8% each, which means that it will negate the happiness reduction effect by overpopulation, but can't actually increase happiness (so if you have only ever had 5 kittens, it will not increase the happiness)
EDIT 2: In reality, it looks like that as of v 0.9.6 Amphitheater does not actually reduce unhappiness by 4.8%. I have 18 kittens for a projected happiness level of 74%, unicorn raises it to 84%, yet 7 amphitheaters only brings it to 93%, about 1.8% each. I'll see if I can find more on this
Comment: Yes, happiness beyond 100% increases your efficiency. I also want to add that when I obtained a certain rare resource from hunting, my 90% happiness goes up to 100%. This happens only once (it doesn't matter how many of that resource I have), but whether this resource increases happiness by 10% or resets it back to 100% I can't say
Comment: It is as you answered. I did go from 88% to 108% with ivory and fur I found from a hunt (but just for less then a minute)
Comment: Ahh, that's why my food production went up when I looted fur or ivory. I thought that it was used to produce catnip or something. But it was the happiness and therefor the effectiveness of the cats that influenced the catnip production.
Comment: It definitely resets when the resources run out.  Also, I believe your first unicorn gives you a straight up +10% to happiness, and that's permanent, as far as I can tell.
Comment: @Mathias711 Well, I didn't want to mention it because that'd have been a surprise to others who want to play, but I was referring to the Unicorn. Well I guess it's 10% increase instead of reset
Comment: Fun fact: your hunters can bring back a unicorn.  That's how I got mine.
Comment: There's also the amphitheater which reduces the effects of overpopulation by 5%. (Before: 18 kittens, 105% happy, 1 theater, after buying another one 106% happy). And then there is the trade post re: bonus duration.  (before: 0 trade posts, Fury -0.18 demand, ivory -0.13 demand.  After: 1 trade posts Furs -0.17 demand, ivory -0.12 demand).
Answer: Looks like the formula changed since the previous answer. This is what i figured out so far:
Happiness start at 100% up to 5 kitten. Some modifies apply, like every special resource (fur, ivory, spice) give +10% (so if you have fur and ivory, you get 120%). Unhappiness is also simply added to this %, so 24% unhappiness (17 kitten) + ivory + fur = 96% happiness.
Spice, Karma and some other things also effect happiness by a flat %.
Amphitheaters affect unhappiness. Unhappiness start at 0% for 5 kitten, and goes up by 2 for every kitten beyond 5. (So 2% at 6 kitten, 20% at 15 etc.) Amphitheaters only affect this modifier in the happiness formula. If you have 10 kittens and 10 amphitheaters, you get 10% unhappiness * 0.52 amphitheaters bonus = 5.2% unhappiness. If you have only fur, that would result in 104% (rounded) happiness.
To be clear: The amphitheaters bonuses are added up, so the formula is  1 - (# of theaters * 0.048) and not 0.952^(# of theaters) as some might expect. This hold true for all buildings, both those that decrease and increase production. A mine gives 20% bonus flat, and a tradepost decreases rare resource consumption flat.  Unhappiness might reach 0 at 21 amphitheaters and rare resource consumption might reach 0 at 25 tradeposts.I cant tell what happened beyond that because i haven't reached that yet - those buildings will be ridiculously expensive at that point.
After 10 tradepost you get diminishing returns, i suspect this i the same for amphitheaters and other buildings that decrease some need. See this post.
Fur, ivory and spice are consumed on use. Once they run out, the happiness bonus you got from them goes away. Unicorns and karma do not get consumed, so once you have these there happiness bonus is permanent.
Comment: Oh, we got a answer [here](http://gaming.stackexchange.com/questions/184936/is-it-possible-to-stop-the-consumption-of-rare-resources). Diminishing returns after 10 tradeposts.
Comment: I already have enough hunters and tradepost's to make my furs and ivory last forever. In fact, i think tradeposts work the same as amphitheaters, there bonuses are added instead of multiplied, so at 25 tradepost's, your rare resource consumption will be 0. - i considered that a bit out of scope, but i guess i can add the way amphitheaters bonuses add.
Comment: 25 tradeshops do NOT give a 0 consumption. I've got 25right now and I do not have enough ivory at the moment. Fur is going well, even some spares for amphitheaters.
Comment: Interesting. Can you share how many cats and ivory consumption you have? (Btw, i do need to send my cats out to hunt all the time to make enough fur and ivory).
Comment: I don't think it is relevant anymore, but: 25 tradeposts, 28 kittens, -0.060 fur consumption, -0.042 ivory and -0.006 spice.
Comment: That's 10 times less then i have on 11 tradeposts. Perhaps 26 pushes you over. I'm running 5 hunters with Composite Bow, Bolas and Hunting Armor to keep up thou.

How does happiness work?


Is a visa to Poland required for a Russian spouse of a Polish citizen?

Question: I am a holder of a Russian passport and my husband is a Polish citizen. We both live and work in the UK. I have received my family member of EEA national card, good for 5 years. We would like to travel to Poland together. On the website of the Polish embassy it clearly states that if we are traveling together, I do not require a visa, however in the embassy they were sure that I do need a visa. Could anyone please clarify?
Answer: You do not need a visa.  The relevant directive is 2004/38/EC.
Article 5, paragraph 2 says:

Family members who are not nationals of a Member State shall only be required to have an entry visa in accordance with Regulation (EC) No 539/2001 or, where appropriate, with national law. For the purposes of this Directive, possession of the valid residence card referred to in Article 10 shall exempt such family members from the visa requirement.

(Emphasis added.)
Article 10, paragraph 1 says:

The right of residence of family members of a Union citizen who are not nationals of a Member State shall be evidenced by the issuing of a document called 'Residence card of a family member of a Union citizen' no later than six months from the date on which they submit the application. A certificate of application for the residence card shall be issued immediately.

You have indicated that your residence card is a "family member of EEA national" card, so I assume that it is in fact the article 10 card that serves to exempt you from the visa requirement.
Comment: IMHO we need a canonical QA for this EU spouse business. Want to take care of it?
Comment: @JoErNanO Yes, I do, but I don't have time to do it now and may not have time before tomorrow.  Did you invite me to chat?  I've never received a chat invitation before and I am not quite sure what to make of it.
Comment: Excellent. I invited you to chat, yes. I wanted to discuss this idea without invading the comment space, but you weren't pingable there. :)
Comment: @JoErNanO was a canonical QA for this ever written?


Print Multiple Selected Documents in View via Web lotus domino

Question: i would like to print selected documents in a view in a way that each document are compiled in a single print. It's like 1 document on page 1 and second document on page 2. I after  i print this documents they would be flagged as printed and then i would be able to export them to a notepad text file. I need to do this in Lotus domino in a web based applications. Answers are greatly appreciated.
Comment: I think you need to elaborate more you do not get native view in the web.In Web every thing is HTML as you know.Are you saying that you want to print the documents that you are seeing in list in a browser for example seeing to email in a browser? You have a list in web and you want to select documents and print them?
Answer: There is no native Lotus Domino print command to print documents selected from the web. This is not an uncommon problem in other technologies either. Once you're using the browser, you are stuck with the browsers capabilities for printing as you would with any other technology, unless the server can generate the content for the print out, say as a PDF or HTML/CSS.
There are a number of ways to do this. The easiest approach I can conceive still requires quite a bit of coding though. There may be other solutions, but it depends what your skill set is.
Others have been trying to solve this problem. You can try this un-tested solution, or here, (but you have to register for free), or here to, or you can "roll your own". Knowing which document has been selected can be found like this
If you have some experience with producing HTML. You can "rollYourOwn" by creating an agent (called "printSelectedDocuments"), that accepts the UNID's of the selected documents from the view, and then constructs a web page that presents the combined documents for printing.
You can can control the HTML output to the browser from an agent using this technique. There is one commercial API you can use, PD4ML. But this still requires a lot of coding, but may take some hassle out of the process.
If you elect to write your own agent, the agent will have to loop through each selected document, and generate the HTML page with the layout of each field you want to print and manage your page breaks. If you really need granular control you can try iText in a java agent and stream back the PDF to the browser. A google around the interwebs indicates that no one has a simple solution.


Strategies for combating ESD and ground plane potential shifts on spacecraft charging?

Question: What are some common design guidelines/practices to combat the electrical effects of spacecraft charging(e.g. ESD, ground plane shift).
Is it to focus on more resilient parts, and reduce resistance/impedence in the ground path, or are there other design guidelines/strategies?
Answer: Regarding grounding on the ISS,

The space station solar arrays operate at 160 VDC. When the arrays are producing power, the station structure will also tend to float to a voltage close to the array voltage. Under these conditions, the space station could be subjected to problems like arcing from its surface to the surrounding environment, or arcing to an astronaut. To avoid these problems, the structure has been grounded with a Plasma Contactor Unit (PCU). To protect the astronauts from shock hazards, the PCU is operated during all spacewalks.
The PCU acts as an electrical ground rod to connect the space station structure to the local environment and harmlessly dissipate the structure charges. Glenn [NASA Glenn Research Centre - ed.] engineers designed, manufactured, tested and installed the hollow cathode assembly, which is the critical component of the PCU. The Hollow Cathode Assembly performs this function by converting a small supply of gas into ions and electrons and discharging this stream to space. The stream carries with it the excess
      electrons that created the surface charge.

From NASA Factsheet PS-00537-0811, "Powering the Future".
My guess is that it would also be operated during docking procedures as well, to minimise ESD and ground potential difference between the ISS and the docking spacecraft.
Also have a look at "Principles of Space Instrument Design", Cruise, A.M et al., Cambridge Uni Press, ISBN 0521451647. There is a chapter on electronics. (I haven't read it yet - only just ordered it today.)
Within manned spacecraft atmospheres, ESD mitigation would most probably be a function of environmental control via humidifiers.
UPDATE 31 Jan 2016:
See Chapter 3, specifically section 3.2 Design Guidelines, of the definitive NASA JPL Space Science & Technology Series book: Guide to Mitigating Spacecraft Charging Effects, Henry B. Garrett, Albert C. Whittlesey, June 2011, 244pp.
Comment: Don't hold your breath on Cruise et al. ESD is mentioned there once or twice only.
Comment: 's okay. It's a well-rounded book.
Comment: @DeerHunter, sounds like I done my money, but I couldn't find much on the market that was affordable, at least.
Comment: I look forward to devouring it when it gets delivered in 3-4 weeks time.
Answer: This document describes NASA's best practices for electrical grounding and ESD handling in rockets.
NASA also studied electrostatic effects on board the Apollo spacecraft, its final design included several measures to reduce static charging, e.g:

inserting a metal mesh in the couches to prevent a static charge from building up as astronauts moved around,
adding discharge resistors to biomedical sensors
Answer: The precautions you take will vary according to the application/orbit etc. It would be prudent to spend some time looking into this; its a big topic and a cause of plenty of modern day failures. Don't rely on advice from this forum! That disclaimer made, here's some ideas to get you started.
Read up about the environment and agency standards for mitigation. Really, you, or someone in your organisation, should be on top of this, its worth asking around. Try this for a list of documents.
As a very rough guide the body of a satellite is usually considered to provide some shielding. Thus it takes high energy electrons to get inside - termed "internal charging" whilst low energy electrons still cause surface charging.
Three very broad failure modes come to mind:
Charging of dielectric material This can occur from surface charging or internal charging. This is hard to do in practice, its everywhere in the harness, though having a slightly leaky insulator can help.
Charging of floating metal  Effects can be larger and more damaging discharges than from a build-up in dielectric though again it can be from surface or internal charging. Real world examples of design features that present a sufficient risk of ESD trigger (in GEO) that they would each warrant a grounding connection:

each layer of aluminised kapton in a blanket

metallic badge on an IC package (or removal of the badge)

unused connector pins

Three phase In locations where there is a high voltage between two conductors a small discharge from a nearby dielectric can produce a brief puff of plasma and thus trigger a sustained discharge between the power rails. Known to cause plenty of damage. I believe its more often associated with surface charging.


Anime movie involving people exploding into monsters?

Question: I've watched an anime movie a few years ago where people are turning into monsters (because of inbalanced magic?). The main character hunts them with the same magic and wears a suit of armor that compresses his body to keep him from turning into a monster himself. I could have sworn it was call Steins;Gate, but it turns out that it is not. Any ideas?
Answer: I think you're looking for Strait Jacket, (2008).

In a world where sorcery and science co-exist, the power of magic
  comes with a price: Humans who do not take proper precautions are
  transformed into horrific demons. Those who destroy these demons - and
  run the highest risk of all - are tactical sorcerists known as Strait
  Jackets.

It was shown as one of the Sci Fi Channel's Ani-Monday features.
Comment: Thank was totally it, thanks! I could not remember it for the life of me!


How are cryptokitties stored on the blockchain?

Question: I'm trying to understand how crypto unique tokens are stored on the blockchain and how is it linked to a graphic of this token. In the case of cryptokitties, each kitty is generated randomly by the smart contract from my understanding. But how is each kitty's image stored on the blockchain? Or is this all controlled by a centralized server and only the smart contract generates random string data to represent a kitty?
Answer: Your second hypothesis is correct. The "genes" are generated on-chain and ownership etc. is also tracked there. Interpreting the raw blockchain data and turning it into a game with a UI is done in a centralized manner with a standard webserver and all that Jazz.
Comment: I'm curious to how are the "genes" generated in a random way. Is there a function that generates random data for this use case?
Comment: Updated my answer to include a link about gene randomness generation. :)
Answer: The images are not stored on the blockchain but the gene string is. The images produced are done via a server. A good example of this is how https://robohash.org/ works. A random string creates a new image.
The gene string is only random to an extent. Each piece of the string represents a crypto kitties characteristics...and during the birthing process, they are modified to still hold most of the characteristics of the parents. There is a function called mixGenes() that leads to a contract called geneScience and unfortunately, the source isn't verified.
Comment: thanks, that really solidified my understanding of identicons which is what cryptokitties is based off of.
Answer: I think you can learn a lot of thath making your own crypto game with cryptozombies.io they have a chapter dedicated to explain your question


Why is it a good idea to teach chess in schools?

Question: There is a "movement" towards teaching chess in school and perhaps introduce chess as its own subject. What are the advantages of teaching chess at schools? In what ways is chess similar to other school subjects (e.g. math)? What "life skills" will the students learn by learning chess?
Comment: The way this question is formulated, I find it hard to accept any answer given so far. Since the question itself assumes it is a good idea to teach chess, and on top of that, in schools. Hmmm.
Answer:
What are the advantages of teaching chess at schools?

That depends. If you like chess, the advantage of having chess classes for free from good teachers (assuming they are good) is that you play your favourite game / learn something you like / have people to play with / all of this is free. It is really good for average chess players (who will be able to find steady income by teaching others).
But knowing this you have to understand that there are people who do not like chess. How will you feel if right now you will be forced  to learn how to play the triangle (kind of funny musical instrument that I by myself find kind of lame). What would you think if your child instead of math/biology/whatever subject you find important would be wasting his time learning triangle/chess which in my opinion will not help his career (assuming he will not become pro triangle / chess player)?
Also forcing a child to do something is not a good way for him to like this something. I enjoy playing chess, but if when I was child I would be forced to go study chess - most probably I would hate this game. There are a lot of places for a person to play chess in his free time enjoying it, so why spoil the fun with compulsory things?

What "life skills" will the students learn by learning chess

What life skills will you gain by swimming / playing the triangle? You will not drown fast in case of flood. You will become a good swimmer. You will learn how to play the triangle and will be able to properly hear/understand music. People can argue that music will make you a better person, swimming will make you more fit, organized, but I am not aware of the proper studies conducted. What skills will you gain by training to solve IQ tests every day? Will you perform well in physics, economy, history, programming? Highly doubtful, and average person who spends 1/10 of your time learning one of these things will perform better. IQ tests will make you a better IQ test solver.
The same way with chess - by learning chess you will learn how to play chess, maybe will learn abstract thinking, thinking ahead and analyzing your options.
I by myself was speaking with some of the top 200 grandmasters and based on their discussion, chess is hard to make money on. You spend thousands of hours on learning preparation, dedication to mastery. And if you are not in top 20 it is really hard to get normal money out of it. Yes, you can publish books, create videos, participate in paid tournaments (which requires a lot of time and effort), but after this amount of time spent in economy you can secure a steady good income.
Comment: The chess mostly teaches you how to play chess. It may be teaching you how to think ahead and abstract thinking, but if you want to learn abstract thinking I think there are other activities which will teach this better. I definitely think that forcing someone to play a game is not a proper way. If a child will like the game, he will find a way to learn. Right now there is huge amount of free chess websites, databases, books and other materials.
Comment: Your answer is food for thought and brings up important points. Your conclusion is that chess only teaches chess and not much other useful skills. Therefore, it should be an optional and not mandatory activity. Right?
Comment: I think you're right. It's really the forcing kids to study chess that's bad. There's probably a tendency that those who are made to play it are also made to study it and learn it which is why a lot of those who are made to play it don't like it. I came up with a similar conclusion on my own before I read this answer that I remember. The brain has its own natural method of thinking which can make them really smart in their own way. I think that for a developing brain, they don't like doing things differently because it's not their natural method. Also for a developing brain, when they're
Comment: stopped from using their own natural method of thinking by being made to study chess, I think their gaining of smartness slows down. I upvoted this answer because I like how you explained that forcing a child to play or study chess is bad.
Answer: The Benefits of Chess in Education
Take a look at The Benefits of Chess in Education (original link) from the Kasparov Foundation, which gives a detailed overview about the benefits of teaching children to play chess.
The document describes among other things the following points:
Academic benefits

Focusing: Children learn to concentrate, because they have to if they want to play good
Visualizing: Children have to image a sequence of actions before it happens
Planning: Children need to plan longer goals and take steps towards them if they want to win
Weighing Options: Children need to weigh options. They are taught not doing the first thing that pops into their mind.
Furhtermore: thinking ahead, analyzing concretely, thinking abstractly, juggling multiple considerations simultaneously

Social benefits
Chess...

... often serves as a bridge to bring together children of any kind
... helps building friendships
... teaches sportsmanship (how to win graciously, how to lose - like in other sports clubs)
Comment: Thanks for the reference! What do you think are the three best points in this document? (If the link is "dead" someday, it would be nice if your answer kept some of the key information). Cheers.
Comment: I will edit my answer with some key points @RauanSagit ;)
Comment: Link is finally dead, replaced it with an archived version.
Answer: The following study holds that chess in schools helps students with low-level gains in intelligence, but no high level gains.  In other words, if I'm correct in my memory of the study it can teach "life lessons" like sports, but with no gains in intelligence versus sports.  So, knowledge of the game would only lead to knowledge in the overlap of fields.  Of course, depending upon your view as to the size of "chess-related" mathematics and computer science this would affect your judgement as to the effectiveness of teaching the game.
The paper is below, judge for yourself.
This flies in the face of what I first thought, but is an interesting take.  As one who publishes in the field of "chess-related" mathematics and played scholastically for 12 years (yes 1st grade through 12th), I say the paper underestimates the vastness of the overlap between mathematics, computer science, and chess (not to mention sociology, philosophy, and other fields).
http://people.brunel.ac.uk/~hsstffg/preprints/chess_and_education.PDF
Comment: One paper hardly makes a scientific consensus
Comment: One would expect if there were high-level gains for the chess player the experiment that shows it would be able to be duplicated.  However, we have many years of mixed results, as pointed out in the paper!
Comment: https://www.chessforsuccess.org/program/study
http://www.championshipchess.net/research.html
http://psmcd.net/otherfiles/BenefitsOfChessInEdScreen2.pdf
One election should not decide my fate for four years, especially when my "vote" was not elected.
Answer: I think a lot of this debate centres around differences between boys and girls and men and women. If we are not allowed IQ or intelligence skills, then what could we possibly  actually mean by "life skills" - sometimes, this is not an easy question either at the chess club,or in the classroom, or outside the pub at closing time.... I could go on. (But I won't). Will they eventually give us a "Life Quotient" and will it be measured by the number of kids you've got, or divorces you've experienced -- will it be means tested???? As I say I could go on.
Answer: Chess is a fun and interesting game to learn and play. It has many benefits for youngsters and thus it is important for the chess in schools to take shape.
Founding my own project, where I teach chess to underprivileged and blind children, I discovered the many benefits of chess for these children.

Chess builds up confidence and makes the players motivated.
It also provides a recreation for the students who otherwise waste their time on television and video games.
It provides a growth medium for the brain and is like a brain gym. In the future, the risk of Alzheimer can be prevented.
Chess makes the player positive and focused.
Chess also helps a player to excel in other school activities.
It develops a strong personality for the player and helps the player to survive strong battles.
It leads to a change in the thinking and pattern recognizing abilities. Improvement in both these areas are visible in most chess players.
Answer: There really cannot be any educational value in just knowing the moves of the pieces, or memorizing how to mate with a Rook, or making random moves on your side while another kid makes random moves on the other.
The justification has to be that a good teacher who is also a chess enthusiast will find the game to be an inexhaustible source of teachable moments, and will use these to convey valuable lessons whether thay are on the syllabus or not. Very few studies of the benefits or lack of benefits ever seem to describe HOW the game was taught, and therfore offer no valid basis for any conclusions


Who should pay the transactions fees?

Question: In traditional transactions, the merchants pay the fees. For bitcoins, it seems that the payer is paying the fees. In pratice, what will happen for a real transaction? If the payer does not enough fees, the transaction won't be confirmed. Or should the merchants include the transaction fees in the price?
Answer: In Bitcoin it is indeed the sender who pays the fees. Also for "real transactions" (with the marginal case of P2SH where one can say that the receiver pays part of the fees upon spending the proceeds of the P2SH transaction).
Indeed, if fees are too low, the transaction is not confirmed. Therefore, the seller typically waits to see whether the transaction is confirmed before delivering anything.
At present the fee is typically less than USD 0.1.
Because the fee necessary for a transaction depends on the length of this transaction (ie its number of inputs, among other things - something determined by the buyer's history), it is hard for the seller to specify a fee that applies to all the expected transactions.The transaction fee can therefore be seen to be only implicitely and approximately included in the price.
Answer: Bitcoin does use a different system to traditional payments.  With Bitcoin the payment is "pushed" to the merchant or other wallet you're sending the coin to so the transaction fee is paid by the payer.
Traditional payment methods use a "pull" system where the payment is taken from the bank account/card of the payer.
Answer: In the case of VISA/MasterCard or other credit/debit card companies, yes, the merchant pays the fees, but not because they want.
The merchants are faced with a dilemma: if they don't support debit/credit card payments, they get less customers. If they want debit/credit payments, they have to sign a contract with these debit/credit card companies, that forces them to comply with some rules. The most important rule is that they cannot pass the credit/debit card fees to the customer, that is, the price for an item should always be the same regardless of the payment method chosen.
In the case of bitcoin, the system is designed, from the start, as a "push" method, as @Marc already said, so the user is the one in charge of paying the fee, in principle. Thing is, as fees raise in the market (because of bitcoin blocks becoming full), customers will try to pay the smaller fees possible, which will make transactions slower to confirm, which is bad for the merchant because it makes the purchase process feel even slower than it's already (as merchants want to wait as many confirmations as possible before considering an item being paid, to avoid double-spend attacks), scaring customers away.
Therefore, it's not unreasonable to think of a not so distant future in which the merchant will use techniques to speed up this process by contributing part of the fee themselves. One of these techniques is called CPFP: Child Pays For Parent, and consists in attaching a higher fee to a subsequent transaction (depending on the first transaction), to attract more miners to process it, and so that both transactions happen faster.

Who should pay the transactions fees?


The Density of Clouds

Question: Clouds are made up of tiny water or ice droplets, depending on temperature. This implies that cloud density is greater than that of dry air. Why don't clouds sink through their surrounding atmosphere rather than float by in a variety of formations?
Comment: Related: http://physics.stackexchange.com/q/153548/2451
Comment: Have a look at this paper. I think it might help you https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/qj.3099
Comment: Answered much better (in my opinion) [here](https://physics.stackexchange.com/a/99702/27542)
Answer: Clouds don't start as clouds.  Most of them start out as humid air, typically from evaporation from a large body of water (say, the Pacific or Atlantic ocean, and most of that between latitudes 30 degrees north and south).
Now humid air is LIGHTER (less dense) than normal air.  This is because gases like to run the same number of moles (number of molecules) per cubic meter, and an H2O molecule weighs less (much less) than an N2, O2 or CO2 molecule.
So, the humid air rises until is cools at higher elevations, forming clouds.  Even at this point, where the microdroplets are denser than the surrounding air, they are not so heavy that they can force their way down.  They are trapped, sort of like grains of sand that get picked up in a sandstorm.  At some point, though, the micro-drops collect into not-so-micro drops and start falling.  As they fall they merge with other moisture and micro-drops to form rain or other precipitation.
To understand how these micro-droplets or micro-crystals can stay up so long, think about hail storms.  We know now that larger hail sizes can be attributed to the initial tiny hail dropping for a distance and then being blown back up by thunderstorm updrafts.  The tiny hail balls go up and down, up and down; and get coated with a new layer of ice each time, until they are finally heavy enough to overcome the updrafts and fall to the earth.  At his point they may be golf ball sized or even larger.  Think of it: billions of balls of hail, each the weight of a snowball, being suspended in space by atmospheric conditions.  After pondering that, it is not hard to believe that micro-droplets can stay up there.
Comment: How does the surrounding air "trap" these microdroplets? And golf-ball sized hail only happens if the hail passes up and down several times, which is exceptional, no? Isn't the normal case that the hail just falls down, meaning that the normal case for cloud droplets would be to just fall?
Comment: " They are trapped, sort of like grains of sand that get picked up in a sandstorm." They should still fall on net even if they're getting blown around. That rate of falling would just be slow.
Answer: The short answer is, it's a colloidal suspension, effectively. From Scientific American:

For particles that are roughly spherical, mass is proportional to the radius cubed ($r^3$); the downward-facing surface area of such a particle is proportional to the radius squared ($r^2$). Thus, as a tiny water droplet grows, its mass becomes more important than its shape and the droplet falls faster. Even a large droplet having a radius of 100 microns has a fall velocity of only about 27 centimeters per second.
  And because ice crystals have more irregular shapes, their fall velocities are relatively smaller.
Upward vertical motions, or updrafts, in the atmosphere also contribute to the floating appearance of clouds by offsetting the small fall velocities of their constituent particles. Clouds generally form, survive and grow in air that is moving upward.
Comment: The quote from Scientific American says nothing about colloidal suspensions, right? It's merely saying that clouds do fall, but they fall slowly, and clouds only exist in places where the wind blows upwards.
Comment: For tiny drops, the drag at fixed speed should be proportional to $r$, not $r^2$.
Answer: Air Density Calculations
Calculations with Vaisala:
http://go.vaisala.com/humiditycalculator/5.0/
All calculations at a pressure of 1013.25mbar
Temperature 20°C rel. humidity 0%:      density:    1.2041 kg/m^3
                100%                1.1936 kg/m^3
Density difference:                         10.5 g/m^3
Temperature 24°C rel. humidity 0%:      density:    1.1879 kg/m^3
                100%                1.1747 kg/m^3
Density difference:                         13.2 g/m^3
Temperature 30°C rel. humidity 0%:      density:    1.1644 kg/m^3
                100%                1.1459 kg/m^3
Density difference:                         18.5 g/m^3
Temperature 34°C rel. humidity 0%:      density:    1.1439 kg/m^3
                100%                1.1264 kg/m^3
Density difference:                         17.5 g/m^3
At the first glance these differences look quite small. But taking into account that fog holds less than 0.3g of condensed water as micron sized droplets it is very large. The amount of invisible water is much higher in that air.
What is the effect of temperature?
20°C vs 24°C:   at 0% rel. hum.     Density difference: 1.2041 – 1.1879 = 16.2 g/m^3
30°C vs 34°C:   at 100% rel. hum.   Density difference: 1.1459 – 1.1262 = 19.7 g/m^3
Density differences because of humidity differences are in the same order of magnitude as with temperature differences and will drive instabilities. Humidity is in so far more important as condensation during up rise will release higher air temperature.
Fluid mechanics:
Density differences cause pressure differences. These cause fluid motion.
Navier – Stokes - Equations do not deal with "potential temperatures" …
Answer: The law of gravity states that the gravitational force exerted on a body has corrolations with its mass. Clouds are groups of very tiny water/ice particles, thus they do fall but their falling velocitz (aka terminal velocity) is much lower for one to see. Also, some atmospheric phenomena such as air streams, pressure difference, deep convection influences the terminal velocity.

The Density of Clouds


What are the risks of buying a used/refurbished computer? How can I mitigate those risks?

Question: I bought a used computer that appears to be wiped clean, but can I be sure?
Comment: Is this question theoretical or practical in nature? I would argue that buying a computer off an unknown third party would be farily low risk if you format (or replace) the storage devices and clear the BIOS. A lot of exploits are discovered through thousands of attack attempts, which is feasible in a digital environment. It would be far less so when operating in the real world, sending out tons of compromised systems hoping for a worthwhile target to buy them.
Comment: This is a very broad question.
Comment: Define "risks."  Are you worried about it being potentially stolen? Counterfeited?  Loaded with spyware?  Packed with C4 to assassinate you?
Comment: A significant number of weird crashes reported to Microsoft are most plausibly explained by people buying used machines that incompetent or unscrupulous former owners have severely overclocked. It might be worth checking whether the observed clock speed matches the manufacturer's recommendation.
Comment: I'd also point out that there is no guaranteed that if you buy a computer new from amazon or a shop it's *actually* 100% new... during assembly, storage, transport etc from the production plant to your home many people could have a chance to stay a significant amount of time unattended with the hardware. Sure, it's probably harder to do and to avoid it being noticed but you are not 100% sure that a new computer wasn't tampered with either.
Answer: It all boils down to which kinds of threats you want to migitate against - a determined attacker has many more options of leaving malware on your computer than just your hard drive, but it's getting increasingly complex to do so. If you're a rocket scientist working on the newest version of ICBMs you have more to fear than a college student who uses their computer for a few class assignments and some of the newest games (but in the rocket scientist's case, you won't be allowed to use self-bought used computers anyway).
Also, it depends a bit on how competent a presumed non-malicious previous owner was in protecting against malware.
And of course, "wipe clean" can mean anything from "deleting personal files, then emptying the wastebasket" to "reinstall the OS" to "completely overwrite the disks with random data/zeroes, then do an install from scratch".
The typical threats you should migitate against are random malware infections that the previous owner caught somehow. Wiping the disks clean, then reinstalling the OS should be enough to get rid of those.
Some computers, mainly laptops, come with a "recovery partition" that allows you to quickly reset the computer to factory condition; I wouldn't use that. First, because a virus might have altered the install files as well to survive a reinstallation; second, these recovery partitions often come with a lot of crapware which the manufacturer installs because they're paid a few dollars by the crapware maker. Better download an installation DVD/USB medium from Microsoft/Ubuntu/whichever OS you're using, and do a fresh install from that.
Besides that, there's many more locations that could have malware, but it's unlikely to find them on your used computer unless you are specifially targeted.

The computer BIOS might be altered to do stuff even before booting, for example, it could have a keylogger trying to steal your passwords, or even keep a few cores of your CPU busy mining bitcoin for the previous owner. It can't hurt to check the manufacturer's website for the newest BIOS update, and flashing the newest version of it, even if that's the version you already (seem to) have. This is already quite paranoid, but still something you can do in a few minutes so doing it won't hurt you much.
The hard drive firmware itself could be altered. Someone published a proof-of-concept a few years ago who installed Linux on the hard drive - no, the computer didn't run Linux; the hard drive itself ran Linux, ignoring the fact that it was supposed to be a hard drive (see http://spritesmods.com/?art=hddhack - on page 6, he adds a backdoor by replacing the root password hash with a known one every time the disk reads /etc/shadow, and on page 7, he boots a linux kernel on the disk itself).
The CPU microcode itself could be altered to do .. stuff, including ignoring any further microcode updates. Unlikely that this could be done without cooperation from Intel, but still a possibility if the NSA is trying to target you. (However, if the NSA were targeting you, "hope he'll buy the spiked used computer we put on ebay for him" wouldn't be their primary strategy, I assume).

So, if you're just a random guy, wiping the hard disks and doing an OS reinstall from scratch provides good protection against the kinds of threats you can reasonably expect.
However, if you have any reason to assume someone is actively targeting you - this includes MDs keeping patient data on their computers, credit card processors, ... - "Just don't buy used computers" is the first thing in a long chain of measures you need to take.
Comment: Good answer. One nitpick: Even if you assume you are actively targeted, I don't see how it would matter whether the computer is used - it would matter whether the attacker knows in advance what you are buying, giving them time to interfere - you can manipulate a new computer just as easily as a used one.
Comment: Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/86370/discussion-on-answer-by-guntram-blohm-what-are-the-risks-of-buying-a-used-refurb).
Comment: However, "intercept the used computer he already bought on ebay" is definitely the sort of thing the NSA might do.
Comment: Overly complicated answer. Personally I feel it would have confused the OP
Answer: Since you are buying a used server, I assume that you aren't the target of a nation state's intelligence services, and that you aren't being targeted with implanted hardware spying modules. All you really need to do is make sure the disk is free of malware.
Load DBAN onto bootable removable media, then boot the server with it and use it to wipe the disks.  That's will get them as clean as you can, without delving deeply into the internals of disk storage.
Comment: Might as well just junk the hard drive they give you and just install an OS on a different one. Hard drives are fairly cheap.
Comment: @kelalaka , BIOS infections are not typically something that some guy buying a used server to run in his basement is likely to encounter. If he is running a Fortune 500 company, he's probably not buying used gear off eBay.
Comment: @KellyBang In which case once the PC goes to you it will no longer be of any interest to the threat actor and if we're talking this sophisticated in terms of spyware they'll have filters to just not act on whatever the PC is then spying on. Also, organizations such as that aren't in the habit of selling off old hardware, most of them properly physically destroy hardware with a crusher for fear of data theft.
Comment: The hard drive is only one component of the computer...albeit the easiest place for an attack (and easiest to defend).
Comment: DBAN may not be enough. [BIOS](https://searchcloudsecurity.techtarget.com/definition/BIOS-rootkit-attack) can also be a problem.
Comment: I've assumed that since he's buying a used machine that he isn't trying to build a system that will secure nuclear launch codes. It's highly unlikely that he'd need to defend against a motherboard implant.
Comment: Unless the company who used to own the server was the target of a nation state's intelligence service...
Answer: Well, depends on how you define secure. Are you being actively targeted by a large organization, such as a conglomerate or a government? Or just scared of criminals taking over your webcam to take photos?
If you are in the first group, congratulations! Assume all electronic devices you use tainted. There's so many ways that these organizations can spy on you without your knowledge, whether it's hardware based or software based. Most electronic devices sold today do not come with a board schematic - you have no way to verify if there isn't an extra component added, such as a keylogger, recording your activity.
On the other hand, if you are just a regular guy with nothing sensitive, I wouldn't worry too much. A simple wipe of the hard drive and a reinstall of whatever operating system you prefer would suffice. Maybe reflash the BIOS if you're really, really paranoid.
Answer: It is pretty simple really, replace the hard drive, update the bios and use bitlocker or other encryption software to lock the drive once you install the OS.
Comment: Whether that's sufficient or not is entirely dependent on the threat model. What if there's a wireless keylogger in the hardware itself?
Answer: The only way I can suggest ensuring said device is clean, if you're that concerned, don't buy it.
I typically will buy refurbished. Why pay top dollar for something that is already outdated the same week it his the sale floor? And with replacement parts such as hard drives getting insanely cheaper just copy down the OEM tag if it's a Microsoft Windows or whatever you need from the OS, transfer it to the new hard drive and good as new.
You could also encrypt and zero the sectors so that if something should happen, like your son or daughter as secretly hacking to change their grades, get busted and the drive confiscated, and unknown to you that the previous owners were hackers but instead of having grades, were robbing banks for Bitcoin...you and your kid don't get hit with charges.
Comment: At first I couldn't see how this answered the question.  Now I see that it does, sort of, but it's not very clear.  Re risks, you name the (astronomically unlikely) risk of getting hit with charges if the drive is confiscated for your kids' grade hacking and evidence of bank robbery by the previous owner is uncovered.  Re mitigation, your answer is to "encrypt and zero the sectors."  Okay, got it.