How come the Aussies have the best explanations for things:
http://www.ph.unimelb.edu.au/~dnj/teaching/160mag/160mag.htm
"The imbalance in the linear charge densities between the positive
metal ions and the moving electrons, measured in the reference frame of
the moving charge, is a result of the Lorentz contraction due to the
relative motions of the nearby charged particle, the electrons flowing
in the wire and the metal ions. This relativistic effect is perhaps
most familiar to us when applied to fast moving objects. Let us see how
fast the electrons are moving in a typical current carrying wire. In a
copper wire the density of copper atoms is about 8.5x10^22 atoms per
cubic centimetre, and hence the density of free electrons is about the
same. In a copper wire with a cross sectional area of 1 square
millimetre and carrying a current of 10 Amps the formula for v given
above shows that the electron velocity is only 0.7 millimetres per
second. This is an extremely small velocity! The Lorentz contraction
for such a small velocity differs from 1 by only 3x10^-24. This
unimaginably small contraction is nevertheless sufficient to cause a
slight imbalance in the positive and negative charge densities of the
wire that causes moving charged particles to feel a magnetic force. "
Terry
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