If the transmitter and receiver are connected together in series with a battery and words are spoken into the transmitter, the little carbon granules are immediately thrown into motion, and being alternately compressed and released cause corresponding changes in the current flowing through the receiver from the battery. The strength of the magnetic pull exerted by the combined permanent magnet and coil of wire varies with each change in the electric current and thus by alternately attracting and repelling the diaphragm causes it to vibrate and emit sounds. This is the general principle of the telephone. The telephones in actual service to-day are complicated with bells, induction coils, condensers and relays, which render them more efficient, but the general principle of the transmission of the speech is the same. I would not advise the young experimenter to attempt to build his own transmitter and receiver except for the knowledge and experience gained thereby. Homemade instruments of this sort are not likely to prove sensitive enough for practical purposes over a long line. A first class telephone receiver can be purchased for 90 cents and an equally good transmitter for $2.25 These, together with an ordinary electric bell can be built into a first class telephone instrument which will give excellent service over a long line. [Illustration: FIG. 96.—Showing how the Push Button is arranged.] Any boys can easily build and install such an arrangement. When completed it will prove invaluable as a source of communication from one part of the house to another, from the house to the barn or between two chums living in the same neighborhood. Figure 93 shows the wooden back of the instrument. It is made from a piece of board 12 inches long and 7 inches wide, by cutting into the shape shown. This back serves to mount the transmitter, receiver, bell, etc. *The Bell* is for the purpose of giving an audible signal so that the person hearing it will know that some one at the other end of the line wishes, to talk. Figure 94 is a front view of the completed instrument. The bell is mounted on the upper part of the back as shown in the illustration. *The Receiver Hook* is shown in detail in Figure 95. It should be cut out of heavy sheet brass and finished in the shape shown. The receiver hook is mounted directly under the bell as shown in Figure 94. The hook should project out to the left beyond the edge of the back. The hook is mounted by a round headed brass screw passing through the hole in the right hand end. [Illustration: FIG. 97.—Circuit showing how to connect two Telephone Stations to the Line.]