A current meter in which the current is measured by the heat it imparts
to a conductor. In one meter a very light helix of mica is poised
horizontally over a conductor, and the whole is enclosed in a case. As
the wire is heated it causes an ascending current of air which rotates
the vane, and the latter moves delicate clockwork which moves indicating
hands. The hotter the wire the more rapidly the air ascends, and
consequently the speed of the vane is proportional to the current,
because the heat of the conductor is proportional thereto.