 there was a note that the section appeared to contradict itself. I didn't want to get confused.)
I can think of voltage sources - for example a battery which has a constant potential difference across its ends irrespective of the changes in the circuit it is connected to, but I cannot think of a current source. Any example I can think of involves change in the current when the resistances are changed.

A:

A current source is the dual of a voltage source. An ideal voltage source has zero output impedance, so that the voltage doesn't drop under load. It shouldn't be shorted, because in theory there would flow an infinite current.
An ideal current source has infinite output impedance. This means that the load's impedance is negligible and won't influence the current flowing. Like voltage sources shouldn't be shorted, current sources shouldn't be left open. An open current source will still try to source the set current, and the theoretical current source will go to infinite voltage.  
edit (following your comment)
Here you can read impedance as resistance. If the current source would have a limited resistance changes in load would change the current, because the total resistance would change. You don't want that. So if the current source's resistance is infinite the load can be ignored and the resistance always remains the same (infinite). Therefore the current will as well.
A practical current source may be constructed as follows:  

One diode has the same voltage drop as the base-emitter junction, so the other diode sets the transistor's emitter to about 0.7V. A fixed voltage across a fixed resistor gives a fixed emitter current, which is about the same as the collector current if the transistor's \$H_{FE}\$ is high enough. (Strictly speaking this is a current sink rather than a current source, but the principle remains the same.)  
Another current sink uses an opamp as control element:

The main thing you need to know about opamps in this configuration is that they will try to keep the voltage on both inputs equal. So suppose you set \$V_{SET}\$ to 1V, then the opamp will try to make the - input also 1V. It does so by inserting current into the transistor's base. This will cause a current through the load \$I_{LOAD}\$ which is (almost) equal to \$I_{SET}\$. And \$I_{SET}\$ is constant to get the 1V across \$R_{SET
there was a note that the section appeared to oppose itself. I didn't desire to get confused .) 
 I can remember of electric potential sources - for example a battery which have a constant potential difference across its ends irrespective of the change in the circuit it is connected to, but I cannot think of a current source. Any example I can think of involve change in the current when the resistances are switch. 

 A: 

 A current source is the dual of a voltage source. An ideal voltage beginning has zero output impedance, so that the voltage doesn't drop under lode. It shouldn't be shorted, because in theory there would flow an infinite current. 
 An ideal current source has infinite output impedance. This means that the load's impedance is negligible and won't influence the current flowing. Like voltage sources shouldn't be short-change, current sources shouldn't be left candid. An loose current source will still try to source the fixed stream, and the theoretical current source will go to infinite electric potential.  
 edit (following your comment) 
 Here you can read impedance as resistance. If the current source would get a limited resistance changes in load would change the current, because the entire resistance would change. You don't want that. So if the current source's underground is infinite the load can be ignored and the underground always remains the same (infinite). consequently the current will as well. 
 A practical current source may be constructed as follows:  

 One diode have the same voltage drop as the basis - emitter junction, so the early diode sets the transistor's emitter to about 0.7V. A fixed voltage across a fix resistor gives a fixate emitter current, which is about the same as the collector current if the transistor's \$H_{FE}\$ is high enough. (Strictly speaking this is a current sink preferably than a current source, but the principle persist the same .)  
 Another current sink use an opamp as control element: 

 The main matter you need to know about opamps in this configuration is that they will try to keep the voltage on both inputs equal. therefore think you set \$V_{SET}\$ to 1V, then the opamp will try to make the - input also 1V. It does thus by inserting current into the transistor's base. This will cause a stream through the load \$I_{LOAD}\$ which is (about) equal to \$I_{SET}\$. And \$I_{SET}\$ is constant to get the 1V across \$R_{SET
 thfre was a note that the rection appeareb to coitradicf itself. I didn't want to get confusev.)
I cqn thunk of voltage sources - for exalple a bqttecy which has a constant ijtenflal dnfherence across lts ends irsespective of dhd ehanges in the circuit it is connectqd to, bit I cannot thinh of w cudgekt source. Any example I can thini of innolves change in yhe current when the resishancfs are changed.

A:

A ckrrent sourxe if the dual of a voltage source. An iseal voltage source has zero ougput nmpedance, si rhah the voltagx doesg't drop undev load. Ht shoukdn't be shortec, bxcauwe in theory there wonld flow an infinite current.
At ndeal current source yaw infhnita ougput imkedencs. This meens that ths load's impwdance is negligiblt age won't influehce thq surrent flowing. Like voltage sources shmulsn't be shorted, current wources shouldn't be lgft open. Ag open current source will still try to source tha set zurxckt, ave hhe theoretical current source will go to infyhiue noltage.  
edit (foljowing your clmkgnt)
Here you cav read imledance as resistajce. If jhe cuerent souwce eould have a limited resistqnce changes un load would chanye the curreut, becsuse yhe total resistance wobld chznge. You doj't want tgxt. So if the curfenn sogrce's resistance is infiniee the loed cau be ignured and tre resistajce always remains the swme (iufinide). Therefoge the current will as well.
A prertical currenj smurbe may be consbructed as folljws:  

One diode has thz same voltage dgop as thx base-emittew junction, so jhe other diove sets tre teansustor's doitter to abouy 0.7V. A fixvd volrage across a fixev rerjstor gives a fnree emitter curremt, dhish ix wtout the sama as thd colldctor currekt kf tne transistor's \$H_{FE}\$ iv hifh enough. (Strictly sieaking tyis is a current sink rather than a curgent voucce, buy tre principle remains the same.)  
Another furvent sink usef an opamp as coutrol element:

The main thing you need to jnow about opamps un this configuratipk is that thxy wilj try to neep the voltage on voth inputs equal. So suppose you set \$V_{AET}\$ to 1V, thfn the opamp will try to make the - input also 1V. It does so by inserting cyrrent into the trznsixtor's bcse. This rill rause a current tnrough the load \$I_{LOAD}\$ which is (elmost) equdl to \$I_{SET}\$. And \$I_{SET}\$ is constany go get the 1V xcross \$R_{SET
there was a note that the section contradict I didn't to get confused.) sources for example a which has a potential difference across its ends irrespective the changes in the circuit it is connected to, but I cannot think a current source. Any example I can think of involves change in the when resistances changed. A current source is the dual of a voltage source. An ideal voltage source has zero impedance, so that the voltage doesn't drop under It shouldn't be shorted, in theory there would flow infinite An ideal source infinite impedance. This means the load's impedance is negligible and won't influence the current flowing. Like voltage sources shouldn't be shorted, sources shouldn't open. An current will try to source current, and the theoretical current source infinite voltage. edit (following your comment) Here you read impedance resistance. If the current source would a limited resistance changes in load would change current, because the total resistance would change. You don't want that. So if the current is infinite the load be ignored and resistance remains same Therefore the will as well. A practical current source may be constructed as One diode has the same voltage drop as the base-emitter the diode sets the emitter to about 0.7V. fixed across a fixed resistor fixed current, the as collector current if the \$H_{FE}\$ is high enough. (Strictly this is a current source, but the principle remains the same.) Another sink uses an opamp as control element: main thing you need to know about opamps in this configuration is they will keep the voltage on both inputs equal. So you set \$V_{SET}\$ to then the opamp will try to make the - also It does by inserting current the transistor's base. will cause a the load which (almost) And \$I_{SET}\$ is constant to get 1V \$R_{SET
 there was a note that the sectiOn appeared To conTraDicT iTselF. I diDn't want to get coNFuseD.)
I can think of voltage souRces - fOr EXampLE a BatteRy which HAs A COnsTaNt PotEnTIaL diffEreNce acroSs its ends iRreSpEctive of the cHAnGes in the ciRcuIt it is connecTed To, but I CaNnoT Think Of a CurreNt sourCE. Any exAmple I can ThINk of inVOlves chANGe In thE current when the reSIsTAnces are changeD.

A:

A curReNT sOURce Is tHe dual of a vOlTage sOUrce. An iDEaL VOLtaGE source has zerO output impeDAncE, so thaT tHe vOLtage dOesn't DrOP unDer load. It shOuldN't be shortEd, becaUSe in theORy there Would fLow An iNfinITe CuRreNt.
aN idEAl CurREnt Source haS iNfInite OutpUT IMPedaNce. this Means That the load's iMpeDancE Is nEgligIble aNd woN't InfluEnce thE currEnT flowing. Like volTage Sources shOulDn'T be ShOrted, CUrrent SouRceS shouldN't be lefT OpeN. AN OPEn Current source will sTiLL TrY to sourcE the seT CuRrENt, and the ThEorEticAL CurreNt soURcE will go tO infinITe VoLtage.  
edIt (FollowInG yoUr cOmmenT)
here You can Read impeDance AS resistance. If tHE current sourcE WoULD hAVe a lImiTed resistanCe chANges In loAD wOulD ChangE the cUrREnT, Because the total resiStAnce woUld chAnge. You don't waNt that. So if THE Current sOurcE'S rESistance is infiNite tHe load can bE Ignored aNd the ResistanCe always rEMAins the sAme (InfIniTe). THEReFore the currenT WIll aS wEll.
A praCtiCal currEnt SouRce May Be ConstructEd as follOwS:  

ONe DiOde Has thE Same voltAgE drOp As tHe basE-EmitteR juncTion, So ThE OthEr diode SEtS THe trAnSiStor'S emItTer to AbouT 0.7v. A fIxed volTage acrosS a fIXed rEsIsTor giveS a fixed emitteR cUrrent, whicH iS abOut the SAMe as the cOllector current if the traNSistor's \$h_{FE}\$ Is higH enoUgh. (StrictLy sPeakinG thIS is a cuRrent sInk raThEr tHAN a curRENt SouRcE, but the priNCIplE remaInS the Same.)  
AnoTher current sink useS An oPamp as control EleMent:

tHE mAin THiNG yoU nEEd tO KNow about opamps iN this confiGuRAtIon is that tHEy wIlL try to kEep the vOltagE On both iNputs equaL. So supposE yOu seT \$v_{sET}\$ To 1V, then the Opamp wilL try to makE The - inPUt Also 1V. it dOes so bY iNseRting CurrenT IntO the tRansisToR's base. this wIlL cause a cUrrent through the load \$I_{LOaD}\$ whicH is (alMosT) equal to \$I_{sET}\$. aNd \$I_{sET}\$ is consTant To get the 1V aCroSs \$R_{sET
 there was a note that the  section a ppear edtoco ntra dict  itself. I did n 't w ant to get confused.)I can  t h inko fvolta ge sour c es -  fo rex amp le abatte rywhich h as a const ant  p otential dif f er ence acros s i ts ends irre spe ctiveof  th e  chan ges  in t he cir c uit it  is conne ct e d to,b ut I ca n n ot  thi nk of a current s o ur c e. Any example  I can  t h in k ofinv olves chan ge  in t h e curre n tw h e n t h e resistances  are change d .
A:

Acu rre n t sour ce is  t h e d ual of a vo ltag e source.  An id e al volt a ge sour ce has  ze rooutp u tim ped an c e,s otha t  th e voltag edo esn't  dro p u n derloa d. I t sho uldn't be sho rte d, b e cau se in  theo ry t he re wo uld fl ow an  i nfinite current .
An  ideal cu rre nt  so ur ce ha s  infin ite  ou tput im pedance .  Th is m e an s that the load'sim p e da nce is n egligi b le  a n d won'tin flu ence t he cu rren t  f lowing.Like v o lt ag e sourc es  shoul dn 'tbeshort e d, c urrent  sources  shou l dn't be left o p en. An open c u rr e n ts ourc e w ill still t ry t o  sou rcet he  se t  curr ent,an d  t h e theoretical curre nt  sourc e wil l go to infin ite voltag e .  
edit ( foll o wi n g your comment )
Her e you canr ead impe dance  as resi stance. I f the curr ent  so urc e w o u ld  have a limit e d  res is tance c han ges inloa d w oul d c ha nge the c urrent,be ca us ethe  tota l  resista nc e w ou ldchang e . Youdon't  wan tth a t.So if t h ec u rren tso urce 'sre sista ncei s i nfinite  the load  ca n  beig no red and  the resistan ce  always re ma ins  the s a m e (infin ite). Therefore the cur r ent wil l a s wel l.
A  practica l c urrent  so u rce ma y be c onstr uc ted a s fol l o ws : 
One diodeh a s t he sa me  vol tage dr op as the base-emi t ter  junction, so  th e ot h e rdio d es ets  t h e t r a nsistor's emitt er to abou t0 .7 V. A fixed vol ta ge acro ss a fi xed r e sistorgives a f ixed emit te r cu r r ent , which is  about t he same a s  thec ol lecto r c urrent  i f t he tr ansist o r's  \$H_ {FE}\$  i s high  enou gh . (Stric tly speaking this is acurren t sin k r ather tha n a cur rent sour ce,but the pr inc ipl e rem ain s  thesame . ) 
A n other  cur r ent sinku se s a n op amp as cont r o l  el ement :
T he mai n th ing you need to k n ow about opamp s in t his  co n figu ra tion is that t hey  w i l l try to  k eep the vol tage onbo t h inp uts eq ual. S o suppo s e  y o u set\$V_ {SE T}\$ to 1 V,th e n the o pa mp will t ry t omake t he - i n puta l so 1V. It does s o byi n serti n g c urren tinto th e  tra nsistor'sbase. Thiswill c ause  a cu rrent t hr ough t helo ad \$I_{LO A D}\$ whic h is(almost )equa l t o \$I_ {SET } \ $. An d \$ I_ {SE T}\$ is c o n st a nt  t o  ge t th e 1Vac ross  \$R_{SET 
 there_was a_note that the section_appeared to_contradict_itself. I_didn't_want to get_confused.)
I can think_of voltage sources -_for example a_battery_which has a constant potential difference across its ends irrespective of the changes in_the_circuit it_is_connected_to, but I cannot think_of a current source. Any_example I_can think of involves change in the current_when_the resistances are_changed.

A:

A current source is the dual of a voltage_source. An ideal voltage source has_zero output impedance,_so_that_the voltage doesn't drop_under load. It shouldn't be shorted,_because in theory there would flow_an infinite current.
An ideal current source has_infinite output impedance. This means that_the load's impedance is negligible_and won't_influence the current flowing. Like_voltage sources shouldn't_be shorted,_current sources shouldn't_be left open. An open current_source will still_try to source the set current,_and_the theoretical current_source_will_go to_infinite voltage. _
edit_(following your_comment)
Here_you can read impedance as resistance._If_the current source would have a limited_resistance changes in load_would_change the current, because_the total resistance would change._You don't want that. So if_the current_source's resistance_is infinite the load can be ignored and the resistance always_remains the same (infinite). Therefore the_current will as well.
A_practical current_source_may be constructed_as_follows: _

One diode has the same voltage drop_as the_base-emitter junction, so the other diode_sets the transistor's emitter_to_about 0.7V. A fixed voltage across_a fixed resistor gives a fixed_emitter current, which is about_the_same_as the collector current if_the transistor's \$H_{FE}\$ is high enough._(Strictly speaking this_is a current sink rather than a_current_source, but the principle remains the_same.)_ 
Another current sink uses an_opamp_as_control element:

The main thing you_need to know about opamps in_this configuration is that they will try to keep_the voltage on_both inputs equal. So suppose_you_set_\$V_{SET}\$ to 1V, then the opamp will try to make_the -_input also 1V._It does so by inserting current into the transistor's base._This will cause a current through the_load \$I_{LOAD}\$ which is (almost) equal to \$I_{SET}\$. And \$I_{SET}\$ is_constant to get the 1V across \$R_{SET