[Ed_Dinning]

Hi Gents. The meters have a voltage and a current coil. the voltage coil is connected between L&N. The current coil is in series with the L wire (usually the 2 outer terminals).
This is the coil that can be shunted with some thick wire as in increasing the range of an ammeter, or it can have a current forced back through it (Kirchoff's law) to reduce the net current flowing and "shave" a few % off the meter reading.
AFAIK readings have been statically checked for sudden changes in reading over the quarter etc.
The magnet round the disc is a brake magnet that simply stops the disc rotating at very light loads. The gap is tiny and the poles are right over the disc, so even modern high power magnets will have little effect unless they encircle the disc.

I was involved with a company that developed a split CT that fitted within the meter, so that if a bypass was added, a mercury coulomb meter would register the imbalance.

Ed