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. A Double-pole Receiver [Illustration: _=Fig. 8=_] Another form of receiver is shown at Fig. 8. This is a double-pole receiver, with the coils of fine wire arranged on the ends of a bent band of steel and located in the cup (A), so that the ends of the magnet are close to the diaphragm (D). Fig. 8 is a sectional view of an assembled receiver, but a good idea can be had from the drawings of the separate parts. The magnet (M) is of steel one-eighth of an inch thick and five-eighths of an inch wide. A blacksmith will make this at a small cost. It should measure two and one-half inches wide, two and one-half inches long, the ends being five-eighths of an inch apart. Thin wooden spools are made from wood or fibre to fit over the steel ends, and are wound with No. 36 silk-insulated wire. A wooden cup, or shell (A), is turned from cherry, maple, or other close-grained wood, and at the back a hole is cut just large enough for the magnet ends to slip through exclusive of the coils wound on them. A plug of wood (A A) is driven between the ends of the magnet to hold them in place. Some shellac on the edges of the hole and the plug will harden and keep the parts in place. The coils (C C) are placed on the magnet ends, and the fine wires are made fast to the binding-posts (E E), the latter being screwed fast to the shell (A). The diaphragm (D) is then arranged in place and held with the cap (B) and the small screws which pass through it and into the shell (A). The Transmitter With any one of these receivers a more complete and convenient telephone can be made by the addition of a transmitter and an induction-coil. Following the invention of the receiver, several transmitters were designed and patented, among them being the Edison, Blake, Clamond, Western Union, and Hunning. The Edison and Hunning are the ones in general use, and as either of them can easily be made by a boy a simplified type of both is shown in Figs. 9 and 11. [Illustration: FIG. 9 FIG. 10 FIG. 11 FIG. 12 SIMPLIFIED TYPE OF TRANSMITTER] Some small blocks of wood, tin funnels, small screws, granulated or powdered carbon, some thin pieces of flat carbon, and a piece of very thin ferrotype plate will be the principal things needed in making a transmitter similar to the one shown in Fig. 9. All that is visible from the outside is a plate of wood screwed to a block of wood, and a mouth-piece made fast to the thin board.