Starting with the four binding posts along the bottom of the back, mark the left hand one B1, mark the next one B2, the third E and the fourth L.
Connect a wire from binding post B1 to the post P1 directly above it. Run the wire from P1 to the brass strip on the push button. Connect binding posts B2 and E together and from E run a wire to the right hand binding post on the bell. Connect a wire from the left hand post on the bell to the lower contact 2. The hook should be connected to binding post L. The upper contact 1 is connected to one terminal of the transmitter. The other terminal of the transmitter goes to binding post P2. The telephone receiver should be connected to the posts P1 and P2 by means, of flexible wire. The contact on the push button should be connected to the upper hook contact 1.
The set is now complete, ready to connect to a similar outfit.
The battery is connected to the binding posts marked B1 and B2. E and L are the line posts to which the wires connecting the two sets are led.
The wire connecting the two outfits should not be smaller than No. 18 B. & S. gauge. No. 14 will prove better if the line is a long one.
The wire should be very carefully insulated. Ordinary porcelain knobs will serve as suitable insulators.
If desirable, only one wire need be used, the circuit being completed by connecting the post E on each instrument to the ground or earth. The gas or water pipes will form the most suitable ground connection.
The number of batteries required will depend upon the length of the line. The actual number necessary to ring the bell properly can be best ascertained by experiment. It should be noticed that the battery connections at each station are reversed so that the batteries will be series. The negative of the battery at one station is connected to E and the positive pole of the other battery at the other station to the similar post.
When the stations are not in use the receivers should be left hanging on the hooks. In order to call the other station, lift the receiver off the hook. This throws the transmitter, receiver and battery into the circuit. The resistance of the receiver and transmitter, however, are so great that they will prevent enough current from flowing to ring the bell at the other station. Pressing the push button will short circuit the transmitter and receiver and allow enough current to flow to ring the bell at the other station. The other person has merely to lift up his receiver in order that the two parties may talk back and forth.
When the receivers are on the hooks both bells are in circuit ready for either party to call up.