Now have a thin, flat spool turned from maple or boxwood to fit over one end of the rod, and wind it with a number of layers of No. 36 copper wire insulated with silk. This is known in the electrical supply-houses as “phone”-receiver insulated wire, and will cost about fifty cents an ounce. One ounce will be enough for two receivers. It should be wound evenly and smoothly, like the strands of thread on a spool, and this may be done with the aid of the winder described on page 58. When the wire is in place a drop of hot paraffine will hold the end so that the wire will not unwind. The ends of this spool-winding should be made fast to heavier wires, which are run through small holes in the tube (A) and project out at the end, as shown at F F. The magnet, with its wire-wound spool on the end, is then pushed through the hole in A until the top end of the rod is slightly below the edges of the cup (E), so that when the metal disk (D) is laid over the cup (E) the space between the magnet and disk, or diaphragm (D), is one-sixteenth of an inch (see Fig. 5). Put some shellac on the magnet, so that when it is in the right place the shellac will dry and hold it fast. The cap (B) holds the disk (D) in place, and protects the spool and its fine wire from being damaged and from collecting dust. After giving the exterior a coat of black paint and a finishing coat or two of shellac, the receiver will be ready for use. The original telephone apparatus was made up of these receivers only--one at each of a line in connection with a battery, bell, push-button, and switch. On a window-casing, or the wall through which the wires passed, a lightning-arrester was arranged and made fast. Using receivers only, it was necessary to speak through the same instrument that one heard through, and for a few years this unhandy method of communication was the only one possible. Then the transmitter was invented. Plan of Installation Many of these single-receiver lines are still in use, and as they require but a small amount of constructive skill a diagram of the wiring and the plan of arrangement is shown in Fig. 6. At the left side, R is the receiver at one end of the line and R 2 that at the other, line No. 1 being a continuous wire between the two receivers. When the boy at R wishes to call his friend at R 2 he uses his push-button (P B), and the battery (B B) operates the electric bell (E B 2) at the other end. In order to have the bell connections operative, the switch (S 2) must be thrown over to the left when the line is “quiet,” while the switch (S) should be thrown to the right. With the switches in this position the boy at either end may call his friend at the opposite end.