ciated with that line are raised to a higher potential than that
which they normally have. The operator in testing a busy line, of course
having previously moved the listening key to the listening position,
closes a path from the test thimble of the jack, through the tip of the
calling plug, through the contacts of the relay _4_, the inside springs
of the listening key, thence through a winding of the induction coil
associated with her set to ground. The circuit thus established allows
current to flow from the test thimble of the jack through the winding of
her induction coil to ground, causing a click in her telephone receiver.
The arrangement of the ringing circuit does not differ materially from
that already described for other systems and, therefore, needs no
further explanation.

[Illustration: Fig. 357. Stromberg-Carlson Multiple Board Circuits]

=Multiple Switchboard Apparatus.= Coming now to a discussion of the
details of apparatus employed in multiple switchboards, it may be
stated that much of the apparatus used in the simpler types is capable
of doing duty in multiple switchboards, although, of course,
modification in detail is often necessary to make the apparatus fit the
particular demands of the system in which it is to be used.

_Jacks._ Probably the most important piece of apparatus in the multiple
switchboard is the jack, its importance being increased by the fact that
such very large numbers of them are sometimes necessary. Switchboards
having hundreds of thousands of jacks are not uncommon. The multiple
jacks are nearly always mounted in strips of twenty and the answering
jacks usually in strips of ten, the length of the jack strip being the
same in each case in the same board and, therefore, giving twice as wide
a spacing in the answering as in the multiple jacks. The distance
between centers in the multiple jacks varies from a quarter of an
inch--which is perhaps the extreme minimum--to half an inch, beyond
which larger limit there seems to be no need of going in any case. It is
customary that the jack strip shall be made of the same total thickness
as the distance between the centers of two of its jacks, and from this
it follows that the strips when piled one upon the other give the same
vertical distance between jack centers as the horizontal distance.

In Fig. 358 is shown a strip of multiple and a strip of answering jacks
of Western Electric make, this being the type employed in the No. 1
sta