In physics, Ohm's law says that current through a wire, $I$, is directly proportional to voltage, $V$, and inversely proportional to resistance, $R$: \[ I=V/R. \]It's also true that resistance is directly proportional to the length of the wire. We have a piece of wire. We pass 12 volts through this wire and measure 100 milliamps of current. If I cut the wire in half and pass 24 volts through it, how many milliamps of current will I measure?
All the factors cancel, and the amount of current is exactly the same, 100 milliamps.
The way I see it is that cutting the resistance in half would double the current.
You also double the voltage, so the current is doubled again. So 4 • 100 mA = 400 mA