Will a transistor always reduce the voltage? I have an NPN transistor that's connected over a 12V adapter. I have tried using a potentiometer to change the resistance of the incomming current. Even if i take the resistance down to 0, I end up with at most 11 Volts when measuring over the magnet.

I have measure the energy without the transistor, in that case it's 12V. Is this expected behaviour of a transistor or is something wrong?
Yes, it is, as both VCE(sat) and the "Collector-Emitter Saturation Voltage vs Collector Current" chart in the datasheet tells you.