wering jack. It happens that the
subscriber on line _3_ requests a connection with line _1_, and the
condition at Section III is that where the operator is about to apply
the tip of the calling plug to the jack of line _1_ to ascertain whether
or not that line is busy. As before stated, when the contact is made
between the tip of the calling plug and the forward contact of the
multiple jack, the operator will receive a click in the ear (by means
that will be more fully discussed in later chapters), this click
indicating to her that line _1_ is not available for connection because
it is already switched at some other section of the switchboard.

=Busy Test.= The busy signal, by which an operator in attempting to make
a connection is informed that the line is already busy, has assumed a
great variety of forms and has brought forth many inventions. It has
been proposed by some that the insertion of a plug into any one of the
jacks of a line would automatically close a little door in front of each
of the other jacks of the line, therefore making it impossible for any
other operator to insert a plug as long as the line is in use. It has
been proposed by others to ring bells or to operate buzzers whenever the
attempt was made by an operator to plug into a line that was already in
use. Still others have proposed to so arrange the circuits that the
operator would get an electric shock whenever she attempted to plug into
a busy line. The scheme that has met with universal adoption, however,
is that the operator shall, when the tip of her calling plug touches the
forward contact of the jack of a line that is already switched, receive
a click in her telephone which will forbid her to insert the plug. The
absence of this click, or silence in her telephone, informs her that she
may safely make the connection.

_Principle._ The means by which the operator receives or fails to
receive this click, according to whether the line is busy or idle, vary
widely, but so far as the writers are aware they all have one
fundamental feature in common. The tip of the calling plug and the test
contact of all of the multiple jacks of an idle line must be absolutely
at the same potential before the test, so that no current will flow
through the test circuit when the test is actually made. The test
thimbles of all the jacks of a busy line must be at a different
potential from the tip of the test plug so that a current will flow and
a click result when