I have 2 questions. I have been thinking about these two things a while, and got no one to ask, so hope to get some info/answeres here:) 1: When an electron is standing still, it produces an electric field (and no magnetic field), right? So when many electrons is moving with a constant speed (a current through a wire), a magnetic field is produced around the current. Why is that? I know when you accelerate electrons you make electromagnetic waves, but electrons moving through a wire is moving with a constant speed (I think). So why is there a magnetic field produced? I have been playing with Maxewll's equations many times, but never really tought of this.. 2: From relativity we can derive the equation for length contraction with the following thought experiment: You place a mirror at the end of a meter-stick, and take the time for the light to bounce back from the mirror back to the front of the meter-stick. You measue the time from 2 different refrence system, one standing still, and one moving with velocity v. My question: why do you have to measure the time it takes the light to reach BACK to the start point? Why isn't it enough to just measure the time it takes the light to reach the end of the meter-stick?? It should produce the same answere, but it will not!