Even though this reply to Jonathan Gillis's question about the magnetic
field produced by light bulb wiring is a little late, I wanted to note
that a compass is used as a current sensor for several experiments in
Chabay and Sherwood's E&M book. A current on the order of .1 A produces
a large enough magnetic field a few mm from the wire to deflect a compass
needle several degrees. (This is assuming the wire is properly oriented,
of course.) As others noted, simply twisting the two wires carrying
current to and from the bulb will produce a very tiny residual field,
whose effect can be further reduced by proper orientation of the twisted
pair.
Steve Luzader
Frostburg State University
sluzader@fre.fsu.umd.edu