ging spring, in contact. The third button shows the springs all in
contact, the condition which exists when ringing a station.

The mechanical construction of the key is shown in Fig. 448. Each button
has a separate frame upon which the springs are mounted. Any one of the
frames with its group of contact springs may be removed without
interfering with either the electrical or the mechanical operation of
the others. This is a convenient feature, making possible the
installation of as few stations as are needed at first, and the
subsequent addition of buttons as other stations are added.

[Illustration: Fig. 448. Push-Button Keys]

The restoring feature is a horizontal metal carriage, in construction
very much like a ladder--one round pressing against each key frame, due
to the tension on the carriage exerted by a single flat spring. The
plunger of each button is equipped with a shoulder, which normally is
above the round of the ladder. When the button is operated, this
shoulder presses against a round of the carriage forcing it over far
enough so that the shoulder can slip by. The upper surface of the
shoulder is flat, and on passing below the pin, allows the carriage to
slip back into its normal position and the pin rests on the top of the
shoulder holding the plunger down. This position places the talking
springs in contact. The ringing springs are open until the plunger is
pressed all the way down, then the ringing contact is made. When the
pressure is released, the plunger comes back to the half-way or talking
position, leaving the ringing contacts open again.

When another button is pressed, the same operation takes place and, by
virtue of the carriage being temporarily displaced, the original key is
left free to spring back to its normal position.

Each station is provided with a button for each other station and a
"home" button. The salient feature of the system is that before a
connection may be established, the button at the calling station
corresponding to the station called and also the home button of the
station called must be depressed, if it is not already down. The home
key at any station, when depressed, transposes the sides of the line
with respect to the talking apparatus. The home key also has a spring
which changes the normal connection of the line at that station from the
negative to the positive side of the talking battery. Unless, therefore,
a connection between two stations is made through the