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 by the mere act, on the part of the user, of removing the receiver from, or placing it upon, a conveniently arranged hook or fork projecting from the side of the telephone casing.

Automatic Operation.  It may be taken as a fundamental principle in the design of any piece of telephone apparatus that is to be generally used by the public, that the necessary acts which a person must perform in order to use the device must, as far as possible, follow as a natural result from some other act which it is perfectly obvious to the user that he must perform.  So in the case of the switch hook, the user of a telephone knows that he must take the receiver from its normal support and hold it to his ear; and likewise, when he is through with it, that he must dispose of it by hanging it upon a support obviously provided for that purpose.