urposes. The talking battery feed is through retardation coils to each
line. When all the hooks are down, each call bell is connected between
the lower common wire and the tip side of the talking circuit individual
to the corresponding station. The ringing buttons at each station are
connected between the tip of the plug at that station and the upper
common wire. As a result, when a person at one station desires to call
another, it is only necessary for him to insert his plug in the jack of
the desired station and press his ringing button; the circuit being
traced from one pole of the ringing battery through the upper common
ringing wire, ringing key of the station making the call, tip of plug,
tip conductor of called station's line, bell of called station, and back
to the ringing battery through the lower common ringing wire.

[Illustration: Fig. 443. Push-Button Wall Set]

_Kellogg Push-Button Type._ Fig. 443 shows a Kellogg wall-type
intercommunicating set employing the push-button method of selecting,
and Fig. 444 shows the internal arrangement of this set.

[Illustration: Fig. 444. Push-Button Wall Set]

_Western Electric System._ The method of operation of the push-button
key employed in the intercommunicating system of the Western Electric
Company is well shown in Fig. 445. When the button is depressed all the
way down, as shown in the center cut of Fig. 445, which represents the
ringing position of the key, contact is made with the line wires of the
station called, and ringing current is placed on the line. When the
pressure is released, the button assumes an intermediate position, as
shown in the right-hand cut, which represents the talking position of
the key and in which the ringing contacts _1_ and _2_ are open, but
contact with the line for talking purposes is maintained. The key is
automatically held in this intermediate position by locking plate _3_
until this plate is actuated by the operation of another button which
releases the key so that it assumes its normal position as shown in the
left-hand cut. When a button is depressed to call a station, it first
connects the called station's line to the calling station through the
two pairs of contacts _4_ and _5_ and then connects the ringing battery
to that line by causing the spring _1_ to engage the contact _2_. The
ringing current then passes through the bell at the called station,
through the back contacts of the switch hook at that station, over one