Take a look at this link, which describes how meters work and how to test them.
http://www.utec.com/PDF/Bul_102.PDF A meter has two coils: the current and the voltage coil. Each of these produces magnetic flux proportional to the respective component, and the composite flux produces _torque_ on the meter disk, proportional to the power flow. This torque would tend to accelerate the meter disk. In order to make the _speed_ of the meter disk proportional to power, some drag is added. The torque reacts against the drag, and you get speed, which you then measure.
If something happened to the drag magnets, the meter would run much more quickly than it should.
-Jon
