the face of the
switchboard, but it is more usual to mount them in strips of five or
ten. A strip of five drops, as manufactured by the Kellogg Switchboard
and Supply Company, is shown in Fig. 248. The front strip on which
these drops are mounted is usually of brass or steel, copper plated,
and is sufficiently heavy to provide a rigid support for the entire
group of drops that are mounted on it. This construction greatly
facilitates the assembling of the switchboard and also serves to
economize space--obviously, the thing to economize on the face of a
switchboard is space as defined by vertical and horizontal dimensions.
These tubular drops, having but one coil, are readily mounted on
1-inch centers, both vertically and horizontally. Sometimes even
smaller dimensions than this are secured. The greatest advantage of
this form of construction, however, is in the absolute freedom from
cross-talk between two adjacent drops. So completely is the magnetic
field of force kept within the material of the shell, that there is
practically no stray field and two such drops may be included in two
different talking circuits and the drops mounted immediately adjacent
to each other without producing any cross-talk whatever.

_Night Alarm._ Switchboard drops in falling make but little noise, and
during the day time, while the operator is supposed to be needed
continually at the board, the visual signal which they display is
sufficient to attract her attention. In small exchanges, however, it
is frequently not practicable to keep an operator at the switchboard
at night or during other comparatively idle periods, and yet calls
that do arrive during such periods must be attended to. For this
reason some other than a visual signal is necessary, and this need is
met by the so-called night-alarm attachment. This is merely an
arrangement by which the shutter in falling closes a pair of contacts
and thus completes the circuit of an ordinary vibrating bell or buzzer
which will sound until the shutter is restored to its normal position.
Such contacts are shown in Fig. 249 at _1_ and _2_. Night-alarm
contacts have assumed a variety of forms, some of which will be
referred to in the discussion of other types of drops and jacks.

[Illustration: Fig. 249. Drop with Night-Alarm Contacts]

_Jack Mounting._ Jacks, like drops, though frequently individually
mounted are more often mounted in strips. An individually mounted jack
is shown in Fig. 250, a