Fig. 239. SCHALLENBERG'S ALTERNATING CURRENT METER.
Meter, Alternating Current.
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.
Meter Bridge.
A form of Wheatstone's bridge in which one lateral pair of arms is
represented by a straight wire. The other pair comprise a known
resistance, and the resistance to be determined. The galvanometer is
connected on one side between the known and unknown resistance. On the
other side its connection is moved back and forth along the straight
wire until the balance is secured and the galvanometer reads zero.
The relative lengths of wire intercepted between the two ends thereof
and the movable galvanometer connection are proportional to the
resistance of these parts and give the necessary data with the one known
resistance for determining the unknown resistance.