A meter for measuring alternating current, as supplied to consumers,
from an alternating current system. Like most commercial meters its only
function is the measurement of quantity; the potential difference is
maintained at a constant figure by the generating plant.

The cut shows the Schallenberg meter. It is simply an alternating
current motor (see Motor, Alternating Current), with air vanes mounted
on its spindle. A main coil passes all the current. Within this is a
second coil complete in itself, and not touching or connecting with the
other. The latter is built up of copper rings. Within the two coils, and
concentric with both is a disc of copper carried by a vertical spindle.
The same spindle carries air vanes, and is free to rotate. As it does so
it moves the indicating machinery.

The current in the outer coil induces one in the inner coil. Owing to
lag, the current in the inner one differs in phase from that in the
outer one, and a rotatory field is produced. The copper disc acquires
induced polarity, and rotates with speed which normally would be in
proportion to the square of the current. But the object of the meter is
to register the current only. The air vanes effect this. The resistance
of the air to their motion causes the rate of rotation to vary directly
as the speed.