I often see on electrical devices, they say how many amps the device uses. But I wonder how can that be? if you do not know the voltage and the resistance (im guessing they do know the resistance) how can they tell you how many amps it pulls? Also doesn't it matter what components are in the circut before the device? What if the circut divides into 100 parallel resistors right before the device, wont the amperage be significantly less? And what if these 100 resistors are in series, wont the voltage then be considerably less, causing a lowering in amperage? The question I am asking, is how can they make a determination of the amps used without knowing the circuit.