indicates that the bell is to be operated only by
negative impulses.

Some specific types of ringers are designed to operate only on a given
frequency of current. That is, they are so designed as to be
responsive to currents having a frequency of sixty cycles per second,
for instance, and to be unresponsive to currents of any other
frequency. Either symbols _E_ or _F_ may be used to designate such
ringers, and if it is desired to indicate the particular frequency of
the ringer this is done by adding the proper numeral followed by a
short reversed curve sign indicating frequency. Thus 50~ would
indicate a frequency of fifty cycles per second.

CHAPTER IX

THE HOOK SWITCH

Purpose. In complete telephone instruments, comprising both talking
and signaling apparatus, it is obviously desirable that the two sets
of apparatus, for talking and signaling respectively, shall not be
connected with the line at the same time. A certain switching device
is, therefore, necessary in order that the signaling apparatus alone
may be left operatively connected with the line while the instrument
is not being used in the transmission of speech, and in order that the
signaling apparatus may be cut out when the talking apparatus is
brought into play.

In instruments employing batteries for the supply of transmitter
current, another switching function is the closing of the battery
circuit through the transmitter and the induction coil when the
instrument is in use for talking, since to leave the battery circuit
closed all the time would be an obvious waste of battery energy.

In the early forms of telephones these switching operations were
performed by a manually operated switch, the position of which the
user was obliged to change before and after each use of the telephone.
The objection to this was not so much in the manual labor imposed on
the user as in the tax on his memory. It was found to be practically a
necessity to make this switching function automatic, principally
because of the liability of the user to forget to move the switch to
the proper position after using the telephone, resulting not only in
the rapid waste of the battery elements but also in the inoperative
condition of the signal-receiving bell. The solution of this problem,
a vexing one at first, was found in the so-called automatic hook
switch or switch hook, by which the circuits of the instrument were
made automatically to assume their proper conditions