common
motor. As no source of commercial power for driving such generators is
absolutely uniform, and since the frequency of the ringing current must
remain very close to a constant predetermined rate, some means must be
employed for holding the generators at a constant speed of revolution,
and this is done by means of a governor shown at the right-hand end of
the shaft in Fig. 413. The principle of this governor is shown in Fig.
414. A weighted spring acts, by centrifugal force, to make a contact
against an adjustable screw, when the speed of the shaft rises a
predetermined amount. This spring and its contact are connected to two
collector rings _1_ and _2_ on the motor shaft, and connection is made
with these by the brushes _3_ and _4_. The closing of the governor
contact serves, therefore, merely to short-circuit the resistance _5_,
which is normally included in the shunt field of the motor. This
governor is based on the principle that weakening the field increases
the speed. It acts to insert the resistance in series with the field
winding when the speed falls, and this, in turn, results in restoring
the speed to normal.

[Illustration: Fig. 414. Governor for Harmonic Ringing Generators]

=Auxiliary Signaling Currents.= Alternating currents, such as those
employed for busy signals to subscribers in automatic systems, those for
causing loud tones in receivers which have been left off the hook
switch, and those for producing loud tones in calling receivers
connected to composite lines, all need to be of much higher frequency
than alternating current for ringing bells. The simplest way of
producing such tones is by means of an interrupter like that of a
vibrating bell; but this is not the most reliable way and it is usual to
produce busy or "busy-back" currents by rotating commutators to
interrupt a steady current at the required rate. As the usual busy-back
signal is a series of recurrent tones about one-half second long,
interspersed with periods of silence, the rapidly commuted direct
current is required to be further commuted at a slow rate, and this is
conveniently done by associating a high-speed commutator with a
low-speed one. Such an arrangement may be seen at the left-hand end of
the multicyclic alternating machine shown in Fig. 413. This commuting
device is usually associated with the ringing machine because that is
the one thing about a central office that is available for imparting
continuous rotary mot