over a greater range than from 970 to 980 to 1,020 to 1,030. In many
cases there may be shafting from which the necessary power can be
taken, and of which the speed is variable only within these limits.
There are several devices by which it has been found possible to
enable a dynamo to maintain a constant electromotive force, even if
the speed of rotation varies over considerable limits. One of these is
that (see illustration) due to Messrs. Trotter & Ravenshaw, and
applicable to shunt or series machines.

In the circuit of the field magnet is placed a variable resistance.
This resistance is thrown in or out by means of a motor device
actuated by an electromotive force indicator. A plunger of soft iron
is suspended from a spring, and hangs within a solenoid of wire, which
solenoid is in connection with the terminals of the dynamo. Any
increase or diminution of the electromotive force causes this iron to
move in or out of the core, and its movement is made to connect or
disconnect the gearing which throws in the field magnet resistance
with a shaft driven by the engine itself. The principle of the
apparatus is therefore that small variations of electromotive force
are made to vary inversely the strength of the magnetic field through
the intervention of a relay mechanism in which the power required to
effect the movement is tapped from the engine.

With the aid of such a governor it is possible to drive a dynamo from
a mill shaft providing the requisite power, but of which the speed of
rotation is not sufficiently uniform to secure alone efficient
regulation of electromotive force. Another device, patented by Mr.
Crompton, is a modification of that method of field magnet winding
commonly known as compound winding. The field magnets are wound over
with two wires, one of which has a high resistance and is arranged as