=122. The Microphone.= If a current of electricity be allowed to pass through a circuit like that shown in Fig. 131, which includes a battery, a Bell receiver, and a microphone, any slight sound near the microphone will be greatly magnified in the receiver. The microphone consists of pieces of carbon so fixed that they form loose contacts. Any slight movement of the carbon causes the resistance to the current to be greatly changed. The rapidly varying resistance allows more or less current to pass, the result being that this pulsating current causes the diaphragm to vibrate. The diaphragm has a constantly varying pull upon it when the carbons are in any way disturbed by the voice, or by the ticking of a watch, etc. This principle has been made use of in carbon transmitters, which are made in a large variety of forms. [Illustration: Fig. 133.] =123. The Carbon Transmitter= does not, in itself, generate a current like the magneto-transmitter; it merely produces changes in the strength of a current that flows through it and that comes from some outside source. In Fig. 132, X and Y are two carbon buttons, X being attached to the diaphragm D. Button Y presses gently against X, allowing a little current to pass through the circuit which includes a battery, D C, and a receiver, R. When D is caused to vibrate by the voice, X is made to press more or less against Y, and this allows more or less current to pass through the circuit. This direct undulating current changes the pull upon the diaphragm of R, causing it to vibrate and reproduce the original sounds spoken into the transmitter. In regular lines, of course, a receiver and transmitter are connected at each end, together with bells, etc., for signaling. [Illustration: Fig. 134.] =124. Induction Coils in Telephone Work.= As the resistance of long telephone lines is great, a high electrical pressure, or E.M.F. is desired. While the current from one or two cells is sufficient to work the transmitter properly, and cause undulating currents in the short line, it does not have power enough to force its way over a long line. To get around this difficulty, an induction coil, Fig. 133, is used to transform the battery current, that flows through the carbon transmitter and primary coil, into a current with a high E. M. F. The battery current in the primary coil is undulating, but always passes in the same direction, making the magnetic field around the core weaker and stronger. This causes an alternating current in the secondary coil and main line. In Fig. 133 P and S represent the primary and secondary coils. P is joined in series with a cell and carbon transmitter; S is joined to the distant receiver. One end of S can be grounded, the current completing the circuit through the earth and into the receiver through another wire entering the earth. [Illustration: Fig. 135.]