, which test circuit
may be traced from the test thimble of the line under test to the tip of
the calling plug, thence through the tip strand of the cord to the
listening key, thence to the outer anvil of the listening key on that
side, through the operator's receiver to ground through the impedance
coil _2_. If, however, the line had already been switched at some other
section by the insertion of either a calling or answering plug, all of
the test thimbles of that line would have been raised to a potential
above that of the ground, by virtue of the battery connected with the
sleeve side of the cord circuit through the retardation coil _1_. If the
operator had made a test of such a line, the tip of her testing plug
would have found the thimble raised to the potential of the battery and,
therefore, a flow of current would occur which would give her the busy
click. The complete test circuit thus formed in testing a busy line
would be from the ungrounded pole of the battery through the impedance
coil _1_ associated with the cord that was already in connection with
the line, thence to the sleeve strand of that cord to the sleeve of the
jack at which the line was already switched, thence through that portion
of the line circuit to which all of the sleeve contacts were connected,
and therefore to the sleeve or test thimble of the jack at which the
test is made, thence through the tip of the calling plug employed in
making the test through the tip side of that cord circuit to the outer
listening key contact of the operator making the test, and thence to
ground through the operator's receiver and the impedance coil _2_. The
resultant click would be an indication to the operator that the line was
already in use and that, therefore, she must not make the connection.

The condenser _3_ is associated with the operator's talking set and with
the extra spring in the listening key _L.K._ in such a manner that when
the listening key is thrown, the tip strand of the cord circuit is
divided and the condenser included between them. This is for the purpose
of preventing any potentials, which might exist on the line with which
the answering plug _P_a was connected, from affecting the busy-test
conditions.

_Operation._ The operation of the system aside from the busy-test
feature is just like that described in connection with the simple
magneto switchboard. Assuming that the subscriber at Station _A_ makes
the call, he turns his hand genera