The action of the instrument is as follows. The current passing through
the wire heats it. This current by Ohm's law is proportional to the
electro-motive force between the terminals. As it is heated it expands
and as it cools contracts, definite expanding and contracting
corresponding to definite potential differences. As the wire expands and
contracts the block or pin c moves back and forth, thus turning the drum
p and cogwheel r one way or permitting it to turn the other way under
the pull of the spring S.
Fig. 345. CARDEW VOLTMETER.
In this construction for a given expansion of the wire the piece c only
moves one half as much. The advantage of using a wire twice as long as
would be required for the same degree of movement were the full
expansion utilized is that a very thin wire can be employed. Such a wire
heats and cools more readily, and hence the instrument reaches its
reading more quickly or is more deadbeat, if we borrow a phraseology
properly applicable only to instruments with oscillating indexes.
In the most recent instruments about thirteen feet of wire .0025 inch in
diameter, and made of platinum-silver alloy is used.