The tip and ring wires of the telephone line, with which the jack is associated, are connected to corresponding terminals
on the rear of the switchboard, and the tip and ring of the line jack are connected respectively to the same terminals, as
explained previously. The sleeve of the line jack is tubular, and makes contact with the sleeve of the plug.
(2) Another type of line jack, is called a cut-off jack. This type of line jack has two auxiliary contacts in addition
to the regular tip, ring, and sleeve contacts. When no plug is in the jack, one auxiliary contact lies against the tip spring of
the jack, and the other against the spring ring. This provides a means of connecting an auxiliary circuit, such as a lamp
circuit, to the tip and ring of the jack through the auxiliary contacts, as will be explained later. When a plug is inserted in
the jack, as shown in the figure, the tip and ring contacts of the jack are spread farther apart, away from the auxiliary
contacts, breaking or cutting off the auxiliary circuit. Although the cut-off jack provides a simple means of connecting an
auxiliary circuit to the tip and ring contacts of the line jack, it has a serious limitation. Since the main and auxiliary
contacts are contained within the jack, they are not readily accessible for adjustment. Special tools must be used, and,
even with such tools, adjustments must be made without possibility of observing the effect produced, unless the jack is
removed from the panel. A means of overcoming this limitation will be discussed later.
(3) Line jacks are mounted individually or in strips of 10 or 20, according to the type of switchboard in which
they are used. The switchboard illustrated in figure 38, for example, has its line jacks mounted in strips of 10.
c. Common battery signal lamps.
(1) The line signal used in common-battery switchboards is usually a small lamp, instead of a ring-down drop,
the drop-shutter mechanism used in local-battery switchboards. A typical signal lamp is illustrated in A, figure 44. It
contains two filament terminals, small metal plates extending along opposite sides of the glass bulb, to which the filament
wires from inside the bulb are soldered. A small wooden or bakelite block, cemented to the rear end of the lamp, supports
and insulates the filament terminals. The glass bulb is tubular in shape to permit easy insertion of the lamp in the
switchboard panel. In B, the lamp is shown in the position of a line lamp. A lamp of similar type is used for supervisory