How does a watt-hour meter work?

There are a couple of kinds of watt-hour meters: one is an electromechanical type (the one with the spinning wheel), and the other is a solid-state type. The solid-state types generally use a current transformer to measure the current produced in the current-carrying conductors, which means that the current carrying conductors need not be connected to the actual measuring device.) The electromechanical type uses an aluminum disc acted upon by two coils. One coil is positioned in such a way as to produce a magnetic flux in proportion to the voltage on the disc, and the other coil produces a magnetic flux proportional to the current. This causes the disc to rotate at a speed proportional to the power (voltage times current) being used. A permanent magnet is also installed in such a way as to exert an opposing force in proportion to the rotating speed of the disc. This causes the disc to slow down and stop when energy stops being used. The aluminum disc is also attached to a worm gear which drives the register.