An induction coil consists of two fiber or hard-wood heads, about one inch square and one-quarter of an inch thick, mounted on the ends of an iron core composed of a bundle of small iron wires about two and one-half inches long. The core should be about five-sixteenths of an inch in diameter. The core is covered with a layer of paper and then wound with three layers of No. 22 B. & S. single-cotton-covered wire. These three layers of wire form the _primary_. The primary is covered with a layer of paper and then the secondary is wound on. The secondary consists of twelve layers of No. 36 B. & S. single-silk-covered magnet wire. It is advisable to place a layer of paper between layers of the secondary winding, and to give each one a coating of shellac. The two secondary terminals of the coil are led out through holes in the fiber head and kept separate from the primary terminals. [Illustration: Fig. 153.—Diagram of Connection for a Telephone System employing an Induction Coil at each Station.] The wiring diagram of a telephone system using an induction coil at each station is shown in Figure 153. The speech sent over a line using an induction coil system is much clearer and more easily understood than that on a line not using such a device. In building telephone instruments or connecting them up, care and accuracy will go a long way towards success. Telephony involves some very delicate and sensitive vibratory mechanical and electrical actions, and such instruments must be very carefully made. ² A transmitter is really a microphone built especially to receive the sounds of the human voice, and operates on the same principle. CHAPTER XII INDUCTION COILS A Medical Coil or shocking coil, as it is properly termed, is nothing more or less than a small induction coil, and consists of a core, a primary winding, a secondary winding, and an interrupter. The principle of an induction coil and that of magnetic induction have already been explained in Chapter V. It might be well for the readers to turn back to pages 89-91 and reread them. The human body possesses considerable resistance, and the voltage of one or two ordinary cells of battery is not sufficient to overcome that resistance and pass enough current through the body to be felt, unless under exceptional conditions. The simplest means employable for raising the voltage of a battery high enough to produce a shock is the medical coil. The first step in making such a coil is to roll up a paper tube, five-sixteenths of an inch in diameter inside, and two and one-half inches long. The outer end of the paper is carefully glued, so that it will not unroll. The tube is filled with pieces of iron wire two and one-half inches long which have been straightened by rolling between two boards. The size of the iron wire may vary from No. 20 to No. 24 B. & S. gauge. Enough should be slipped into the tube to pack it tightly and admit no more.