458 ELECTRICAL APPARATUS

rings at speeds of 10,000 to 15,000 ft. per minute, leads to high
losses and correspondingly low machine efficiency, high tempera-
ture rise, and rapid wear of the brushes and collector rings,-and
this has probably been the main cause of abandoning the develop-
ment of the unipolar machine for steam-turbine drive.

A contributing cause was that, when the unipolar steam-tur-
bine generator was being developed, the days of the huge direct-
current generator were over, and its place had been taken by
turbo-alternator and converter, and the unipolar machine offered

FIG. 227.—Unipolar motor meter.
no advantage in reliability, or efficiency, but the disadvantage
of lesser flexibility, as it requires a greater concentration of direct-
current generation in one place, than usually needed.
252. The unipolar machine may be used as motor as well
as generator, and has found some application as motor meter.
The general principle of a unipolar meter may be illustrated by
Pig. 227.
The meter shaft, A} with counter, F, is pivoted at P, and carries
the brake disk and conductor, a copper or aluminum disk, JD, be-
tween the two poles, N and S, of a circular magnet. The shaft, A,
dips into a mercury cup, C, which is insulated and contains the
one terminal, while the other terminal goes to a circular mercury
trough, 6. An iron pin, jB, projects from the disk, D, into this
mercury trough and completes the circuit.