f forms. The well-known type of relay employed
in telegraphy would answer the purpose well but for the amount of room
that it occupies, as it is sometimes necessary to group a large number
of relays in a very small space. Nearly all present-day relays are of
the single-coil type, and in nearly all cases the movement of the
armature causes the movement of one or more switching springs, which are
thus made to engage or disengage their associated spring or springs. One
of the most widely used forms of relays has an L-shaped armature hung
across the front of a forwardly projecting arm of iron, on the
knife-edge corner of which it rocks as moved by the attraction of the
magnet. The general form of this relay was illustrated in Fig. 95.
Sometimes this relay is made up in single units and frequently a large
number of such single units are mounted on a single mounting plate. This
matter will be dealt with more in detail in the discussion of
common-battery multiple switchboards. In other cases these relays are
built _en bloc_, a rectangular strip of soft iron long enough to afford
space for ten relays side by side being bored out with ten cylindrical
holes to receive the electromagnets. The iron of the block affords a
return path for the lines of force. The L-shaped armatures are hung over
the front edge of this block, so that their free ends lie opposite the
magnet cores within the block. This arrangement as employed by the
Kellogg Company is shown in two views in Figs. 323 and 324.

[Illustration: Fig. 323. Strip of Relays]

[Illustration: Fig. 324. Strip of Relays]

A bank of line relays especially adapted for small common-battery
switchboards as made by the Dean Company, is shown in Fig. 325.

[Illustration: Fig. 325. Bank of Relays]

=Jacks.= The jacks in common-battery switchboards are almost always
mounted in groups of ten or twenty, the arrangement being similar to
that discussed in connection with lamp strips. Ordinarily in
common-battery work the jack is provided with two inner contacts so as
to cut off both sides of the signaling circuit when the operator plugs
in. A strip of such jacks is shown in Fig. 326.

[Illustration: Fig. 326. Strip of Cut-Off Jacks]

Ringing and listening keys for simple common-battery switchboards differ
in no essential respect from those employed in magneto boards.

[Illustration: Fig. 327. Details of Lamp, Plug, and Key Mounting]

=Switchboard Assembly.= The general assembly of the