ere the
subscriber's station at the left is indicated in the simplest of its
forms. It is well to repeat here that in all common-battery manual
systems, the subscriber's station equipment, regardless of the
arrangement or type of its talking and signaling apparatus, must have
these features: First, that the line shall be normally open to direct
currents at the subscriber's station; second, that the line shall be
closed to direct currents when the subscriber removes his receiver from
its hook in making or in answering a call; third, that the line
normally, although open to direct currents, shall afford a proper path
for alternating or varying currents through the signal receiving device
at the sub-station. The subscriber's station arrangement shown in Fig.
307, and those immediately following, is the simplest arrangement that
possesses these three necessary features for common-battery service.

[Illustration: Fig. 307. Direct-Line Lamp]

Considering the arrangement at the central office, Fig. 307, the two
limbs of the line are permanently connected to the tip and sleeve
contacts of the jack. These two main contacts of the jack normally
engage two anvils so connected that the tip of the jack is ordinarily
connected through its anvil to ground, while the sleeve of the jack is
normally connected through its anvil to a circuit leading through the
line signal--in this case a lamp--and the common battery, and thence to
ground. The operation is obvious. Normally no current may flow from the
common battery through the signal because the line is open at the
subscriber's station. The removal of the subscriber's receiver from its
hook closes the circuit of the line and allows the current to flow
through the lamp, causing it to glow. When the operator inserts the plug
into the jack, in response to the call, the circuit through the lamp is
cut off at the jack and the lamp goes out.

This arrangement, termed the direct-line lamp arrangement, is largely
used in small common-battery telephone systems where the lines are very
short, such as those found in factories or other places where the
confines of the exchange are those of a building or a group of
neighboring buildings. Many of the so-called private-branch exchanges,
which will be considered more in detail in a later chapter, employ this
direct-line lamp arrangement.

[Illustration: Fig. 308. Direct-Line Lamp with Ballast]

_Direct-Line Lamp with Ballast._ Obviously, however, t