UNIPOLAR MACHINES 451

By bringing the conductor, C, over the end of the magnet close
to the shaft, as shown in Fig. 216, the peripheral speed of motion
of brush, J32, on its collector ring can be reduced. However, at
least one brush, Bi, in Fig. 216, must bear on a collector ring
(not shown in Figs. 215 and 216) at full conductor speed, because
the total magnetic flux cut by the conductor, C, must pass through
this collector ring on which BI bears. Thus an essential char-
acteristic of the unipolar machine is collection of the current from
the periphery of the revolving conductor, at its maximum speed.
It is the unsolved problem of satisfactory current collection from
high-speed collector rings, at speeds of two or more miles per

FIG. 216.—Diagrammatic illustration of unipolar machine with one high-
speed collector.
minute,, which has stood in the way of the commercial intro-
duction of unipolar machines.
Electromagnetic induction is due to the relative motion of con-
ductor and magnetic field, and every electromagnetic device is
thus reversible with regards to stationary and rotary elements.
However, the hope of eliminating high-speed collector rings in
the unipolar machine, by having the conductor standstill and
the magnet revolve, is a fallacy: in Figs. 215 and 216, the con-
ductor, C, revolves, and the magnet, NS, and the external circuit,
D, stands stilL The mechanical reversal thus would be, to have
the conductor, C, stand still, and the magnet, NS, and the external
circuit revolve, and this would leave high-speed current collection.
Whether the magnet, NS, stands still or revolves, is immaterial
in any case, and the question, whether the lines of force of the
magnet are stationary or revolve, if the magnet revolves around
its axis, is meaningless. If, with revolving conductor, C, and
stationary external circuit, D, the lines of force of .the magnet
are assumed as stationary, the induction is in C, and the return
circuit in D\ if the lines of force are assumed as revolving, the