central-office battery which energizes a signal associated with the
line at the central office. This use of the hook switch in the case of
the common-battery telephone is a good illustration of the principle
just laid down as to making all the functions which the subscriber has
to perform depend, 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