The wiring is comparatively simple, and may be easily followed if the description and plan are constantly consulted when setting up the line. R and R 2 are the receivers, T and T 2 the transmitters, C 1 and C 2 the batteries, E B and E B 2 the buzzers or bells, P B and P B 2 the push-buttons, and L S and L S 2 the lever-switches. For convenience of illustration the induction-coils are separated. The primary coil (P C) is indicated by the heavy spring line and the secondary coil (S C) by the fine spring line. When the line is “dead” both receivers are hanging from the hooks of the lever-switches. If the boy at the left wishes to call the boy at the right he lifts the receiver (R) from the hook (L S) and presses the button (P B). This throws the battery (C 1 C 1 C 1) in circuit with lines Nos. 1 and 2, and operates the buzzer (E B 2). When the boy at the right lifts his receiver (R 2) from the hook (L S 2), the bell circuit is cut out and the ’phone circuit is cut in. When the lever-switches are drawn up against the contact-springs (A, B, and C and A A, B B, and C C), both batteries are thrown into circuit with the transmitters at their respective ends through the primary coils (P C and P C 2). By inductance through the secondary coils (S C and S C 2), lines Nos. 1 and 2 are electrified, and when the voice strikes the disks in the transmitters the same tone and vibration is heard through the receivers at the other end of the line. While conversation is going on the batteries at either end are being drawn upon or depleted; but as soon as the receivers are hung on the hooks and the lever-switches are drawn away from the contact-springs, the flow of current is stopped. The buzzers or bells consume but a small amount of current when operated, and in dry cells the active parts recuperate quickly and depolarize. The greatest drain on a battery, therefore, is when the line is closed for conversation. An Installation Plan A simple manner in which to install this apparatus in boxes is shown in Fig. 22. The box is depicted with the front opened and with the receiver hanging on the hook. When the lever-switch (L S) is down it rests on the contact-spring (A), thus throwing in the bell circuit. When the boy at the other end of the line pushes the button on his box it operates the buzzer (E B). This can be understood by following with a pointer the wires from the buzzer to the outlet-posts (Nos. 1 and 3) at the bottom of the wall-plate. [Illustration: _=Fig. 22=_] When the receiver (R) is lifted from the hook (L S), it cuts out the bell circuit and cuts in the telephone circuit, through the spring-contacts (B and C). This circuit may easily be followed through the wires connecting transmitter, receiver, induction-coil, and batteries. The heavy lines leading out from the induction-coil are the primary coil wires, and the fine hair lines are those forming the secondary coil. The medium lines are those that connect the binding-posts, batteries, and lines.