nd, as far as possible, on acts which his common sense
alone tells him he must do. Thus, in the common-battery telephone the
subscriber has only to place the receiver at his ear and ask for what
he wants. This operation automatically displays a signal at the
central office and he does nothing further until the operator
inquires for the number that he wants. He has then nothing to do but
wait until the called-for party responds, and after the conversation
his own personal convenience demands that he shall dispose of the
receiver in some way, so he hangs it up on the most convenient object,
the hook switch, and thereby not only places the apparatus at his
telephone in proper condition to receive another call, but also
conveys to the central office the signal for disconnection.

Likewise in the case of telephones operating in connection with
automatic exchanges, the hook switch performs a number of functions
automatically, of which the subscriber has no conception; and while,
in automatic telephones, there are more acts required of the user than
in the manual, yet a study of these acts will show that they all
follow in a way naturally suggested to the user, so that he need have
but the barest fundamental knowledge in order to properly make use of
the instrument. In all cases, in properly designed apparatus, the
arrangement is such that the failure of the subscriber to do a certain
required act will do no damage to the apparatus or to the system, and,
therefore, will inconvenience only himself.

Design. The hook switch is in reality a two-position switch, and
while at present it is a simple affair, yet its development to its
high state of perfection has been slow, and its imperfections in the
past have been the cause of much annoyance.

Several important points must be borne in mind in the design of the
hook switch. The spring provided to lift the hook must be sufficiently
strong to accomplish this purpose and yet must not be strong enough to
prevent the weight of the receiver from moving the switch to its other
position. The movement of this spring must be somewhat limited in
order that it will not break when used a great many times, and also it
must be of such material and shape that it will not lose its
elasticity with use. The shape and material of the restoring spring
are, of course, determined to a considerable extent by the length of
the lever arm which acts on the spring, and on the space which is
available for t