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].
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.
In [Fig. 10] an interior section is shown, which when once understood will be found extremely simple. The block (A) is of pine, white-wood, birch, or cherry, and is two inches and three-quarters square and five-eighths or three-quarters of an inch thick. A hole seven-eighths of an inch in diameter is bored in the centre of this block, half an inch deep, and a path is cut at the face of the block one inch and a half in diameter and one-eighth of an inch deep. Be careful to cut these holes accurately and smoothly, and if it is not possible to do so, it would be well to have them put in a lathe and turned out.
The face-plate (B) is two inches square, with a three-quarter-inch hole in it, and the under-side is cut away for one-eighth of an inch in depth and one inch and a half in diameter. The object of these depressions in block A and face-plate B is to give space for the diaphragm (D) to vibrate when the voice falls on it through the mouth-piece (C).
From carbon one-eighth of an inch in thickness two round buttons are cut measuring three-quarters of an inch across. A small hole is bored in the centre of each button, and one of them is provided with a very small brass screw and nut, as shown at F F. One side of the button-hole is countersunk, so that the head of the screw will fit down into it and be flush with the face of the carbon. With a small three-cornered or square file cut the surface of the buttons with criss-cross lines, as shown at F F. When the buttons are mounted in the receiver these surfaces will face each other. Cut a small washer from felt or flannel, and place it in the bottom of the hole in block A. Line the side of the hole with a narrow strip of the same goods; then place the button (F F) in the hole, pass the screw through the hole and through the block (A), and make it fast with the nut, as shown at F. Place a thin, flat washer under the nut, and twist a fine piece of insulated copper wire between washer and nut for terminal connections, taking care that the end of the wire under the nut is bare and bright, so that perfect contact is assured. Since the practice of telephony involves such delicate and sensitive vibratory and electrical phenomena, it is best to solder all joints and unions wherever practicable, and so avoid the possibility of loose connections or corrosion of united wires.
From very thin ferrotype plate cut a piece two inches square, and at the middle of it attach the other carbon button by means of a small rivet which you can make from a piece of copper wire. Or a very small brass machine screw may be passed through the button and plate; then gently tapped at the face of the plate to rivet it fast, as shown at E. Lay the block down flat and partly fill the cavity with carbon granules until the button is covered. Do not fill up to the top of the hole. Over this lay the disk (D), so that the carbon button at the under side of it will fit in the top part of the hole between the sides of felt or flannel. Make the disk fast to the block (A) with small pins made by clipping ordinary pins in half and filing the ends.