Symbols _A_ or _B_ may be used for designating any ordinary polarized
ringer. Symbols _C_ and _D_ are interchangeably used to indicate a
biased ringer. If the bell is designed to operate only on positive
impulses, then the plus sign is placed opposite the symbol, while a
minus sign so placed 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