Why did a 12V battery affect my motor more than two 9v batteries in series? In class, we made a coil of wire and used it to make a simple electric motor with a magnet. I put two square 9v batteries in series for a total of 18v to power the motor. 
I then used a larger 12v battery ( probably about 2x4x4 inches ) to power it. The 12v caused the motor to spin a lot faster and even cause some arcing.
Why is this so? Why would a lower voltage cause the motor to spin faster? Does it have to do with internal resistance of the battery? What other factors are at play here?
Sorry, but I don't know the specific 12v battery, that's why I described it's dimensions. The "motor" was just hand made by coiling about 28 AWG wire a couple of times.
 A: you are right, the internal resistance of the two 9V cells was big enough that when you hooked them up to the motor, you were probably dropping more than 6 volts across that resistance.  
A: A battery's ability to deliver power depends not only on its voltage, but also its ability to deliver current.  Your don't give enough information to know what the motor requires or details on the batteries, but I suspect that the 12v battery can deliver more power that the 9v batteries.
Another interesting experiment to perform is hooking the two 9v batteries in parallel and trying again. In parallel, you would get double the current assuming 9v is enough to power the motor.  The power delivered is $volts*amps$.
Car batteries are pretty much all 12v, but advertisers list cold cranking amps, because that is more indicative of their ability to deliver power. 
A: The motor was the only component in the circuit so the resistance in the motor increased because it had more voltage. The motor requires higher current to spin faster. IT does look like this L~I.
