When the bladders are ready, cut off the necks, and also remove about one-third of the material, measuring from end to end. Soak the bladders in warm water until they become soft and white. Stretch them, loosely but evenly, over the opening in the boards, letting the inside of the bladder be on top, and tack them temporarily all around, one inch from the edge of the opening. Test for evenness by pushing down the bladder at the middle. If it stretches smoothly and without wrinkles it will do; otherwise the position and tacks must be changed until it sets perfectly smooth.
The bladder must now be permanently fastened to the board by means of a leather band half an inch wide and tacks driven closely, as shown in Fig. 1. With a sharp knife trim away the rough edges of the bladder that extend beyond the circle of leather. Attach a piece of the fine wire to a button, as shown in Fig. 2, and pass the free end through the centre of the bladder until the button rests on its surface. Then fasten an eight-pound weight to the end of the wire and set in the sun for a few hours, until thoroughly dry, as shown at Fig. 3.
When both drums are complete, place one at each end of a line, and connect the short wires with the long wire, drawing the latter quite taut. The course of the main wire should be as straight as possible, and should it be too long it may be supported by string loops fastened to the limbs of trees, or suspended from the cross-piece of supports made in the form of a gallows-tree or letter F. To communicate it will be necessary to tap on the button with a lead-pencil or small hard-wood stick. The vibration will be heard at the other end of the line and will attract attention.
By speaking close to the bladder in a clear, distinct tone, the sound will carry for at least a quarter of a mile, and the return vibrations of the voice at the other end of the line can be clearly recognized.
The principal parts of the modern telephone apparatus are the transmitter, receiver, induction-coil, signal-bell, push-button, batteries, and switch. The boxes, wall-plates, etc., etc., are but accessories to which the active parts are attached.
The first telephone that came into general use was the invention of Graham Bell, and the principle of his receiver has not been materially changed from that day to this, except that now a double-pole magnet and two fine wire coils are employed in place of the single magnet and one coil. A practical form of single magnet receiver that any boy can easily construct is shown in Fig. 4, and Fig. 5 is a sectional drawing of the receiver drawn as though it had been sliced or sawed in two, from front to rear.