Voltmeter, Battery.
A voltmeter for use in running batteries. In one form (Wirt's) it is
constructed for a low range of voltage, reading up to two and a half
volts and having exactly one ohm resistance, thus giving the battery
some work to do.
Voltmeter, Cardew.
A voltmeter in which the current passing through its conductor heats
such conductor, causing it to expand. Its expansion is caused to move an
index needle. By calibration the movements of the needle are made to
correspond to the potential differences producing the actuating currents
through it. The magnetic action of the current plays no part in its
operation. It is the invention of Capt. Cardew, R. E.
The construction of the instrument in one of its most recent forms is
shown in the cut. On each side of the drum-like case of the instrument
are the binding screws. These connect with the blocks m and n. To these
the fine wire conductor is connected and is carried down and up over the
two pulleys seen at the lowest extremity, its centre being attached to
c. From c a wire is carried to the drum p, shown on an enlarged scale on
the left of the cut. A second wire from the same drum or pulley connects
to the spring S. The winding of the two wires is shown in the separate
figure of c, where it is seen that they are screwed fast to the
periphery of the little drum, and are virtually continuations of each
other. By the screw A the tension of the spring S is adjusted.
On the shaft of the little drum p is a pinion, which works into the
teeth of the cog-wheel r. The shaft of r is extended through the dial
of the instrument, and carries an index. The dial is marked off for
volts; g g and h h are standards for carrying the pulleys.