In the reverse manner, when the switch (S) is thrown to the right, the boy at the opposite end rings the bell (E B) by pressing on the button (P B 2), first throwing switch S 2 over to the right. If the boy at the left is calling up the boy at the right, the switch (S) should be thrown to the left, and he keeps ringing until the other operator throws switch S 2 over to the right. If now he has the receiver (R) up to his ear he can hear the vibration of the bell (E B 2) ringing through the receiver (R) at his end of the line. But when the boy summoned to R 2 takes up the receiver and places it to his ear, he throws switch S 2 over to the right side, and the boy at R leaves switch S over on the left side. This brings the lines into direct connection with the receivers in series. Be careful, when setting up this line, to have the batteries (B B) in series with B 2 B 2; otherwise there would be counter-action. The carbon of one cell should be connected with the zinc of the next cell, and so on.
Another receiver is shown at Fig. 7. The tube (A) and the cup are turned from one piece of wood, and the cap (B) from another piece. The length of the receiver is five inches, and the cap is two inches and a half across. The shank, or handle, through which the magnet is passed measures one inch and a quarter in diameter.
These wood parts will have to be made by a wood-turner; and before the long piece is put in a lathe the hole, three-eighths of an inch in diameter, should be bored. It must be done carefully, so that the wood shell will be of even thickness all around the hole. Also two small holes should be made the entire length of the handle, through which the wires leading from the coil to the binding-posts may pass.
The spool for the fine insulated wire coil is turned from box-wood or maple, and wound as described in chapter iv., on Magnets and Induction-coils. Small binding-posts (F F) with screw ends should be driven down into the holes at the end of the handle and over the bare ends of the wires that project out of the holes. The magnet (M) is three-eighths of an inch in diameter, and is provided with the spool and coil (C) at the large end of the receiver.
The disk (D) is of very thin iron, and is held in place by the cap (B) and four small brass screws driven through the edge of B and into the cup end of A. A screw-eye should be driven into the small end of the receiver from which it may hang from a hook. If a double hook and bar is employed the receiver will hang in the fork, being held there by the rim of wood turned at the small end of A.