Figuring out some numbers for a mini pump I'm just now getting into electronics (I don't know why I waited so long) and I'm just figuring out some stuff.  I picked up an Arduino kit from Vilros.com and a few other parts for my project.

I'm currently trying to work on a project that is basically a humidor keeper.  Living in So.California, it's extremely dry here and I would like a perfectly controlled environment for my humidor.  I purchased a few things like some humidity sensors (as an input obviously), a humidifier (basically a water canister with a sponge and tubes) some tube splitters and twist valves and most importantly, a small air pump.

The small air pump is what I'm having problems with at the moment.  This is the pump. Obviously it's a 3v pump.  I will be using the supplied 9v battery cable when the project is ready, so I understand I'll have to use resistors to bring the voltage down so I don't burn out the pump.  As I said, I'm new to this, so I'm trying to figure out ohms law to find out what kind of resistor(s) I will need.  The problem is, I don't know the amperage.  Where would I find this?

I looked at the details for the pump and what looks like amps (or milliamps) is really just the product number.  Can anyone help me out?

Right now, I have my equation as r=6v / ?i  But where do I find the current?
There are two serious problems with your approach, which you will need to change in order to end up with a sound solution.


If by "9v battery cable" you mean that you intend to use a 9v battery, this is highly unsuitable for motor applications given its low power storage capacity and high internal impedance - without extreme care to use sleep modes, it probably can't even run the Arduino alone for a week.  It would be much more appropriate to use a larger capacity lower voltage source - 4 or 5 C or D cells for example.  A power supply could be another option, though given the water involved, be very careful to pick an isolated one and use a ground fault protected outlet.
Resistors are a poor way of reducing the supply voltage to a motor, as they are not only wasteful, but the applied power will depend too much on the load.  A professional solution would use pulse with modulation to apply the supply voltage at a reduced duty cycle - something you can try with one the various motor shield, but you will need to be careful not to overheat the motor by using too high a duty cycle.  


Probably your best bet would be to buy a pump with a 6 or 7v motor (as is showing up in the associated products at the bottom of that page), or even replace the standard-form-factor motor in that pump with a higher voltage one.  A 7v or so supply voltage can then be a compromise solution for running both the motor and the arduino.