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, and a strip of ten jacks in Fig. 251.  In such a strip of jacks, the strips supporting the metallic parts of the various jacks are usually of hard rubber reinforced by brass so as to give sufficient strength.  Various forms of supports for these strips are used by different manufacturers, the means for fastening them in the switchboard frame usually consisting of brass lugs on the end of the jack strip adapted to be engaged by screws entering the stationary portion of the iron framework; or sometimes pins are fixed in the framework, and the jack is held in place by nuts engaging screw-threaded ends on such pins.

[Illustration:  Fig. 250.  Individual Jack]

[Illustration:  Fig. 251.  Strip of Jacks]