If desirable, only one wire need be used, the circuit being completed by connecting the post E on each instrument to the ground or earth. The gas or water pipes will form the most suitable ground connection. The number of batteries required will depend upon the length of the line. The actual number necessary to ring the bell properly can be best ascertained by experiment. It should be noticed that the battery connections at each station are reversed so that the batteries will be series. The negative of the battery at one station is connected to E and the positive pole of the other battery at the other station to the similar post. When the stations are not in use the receivers should be left hanging on the hooks. In order to call the other station, lift the receiver off the hook. This throws the transmitter, receiver and battery into the circuit. The resistance of the receiver and transmitter, however, are so great that they will prevent enough current from flowing to ring the bell at the other station. Pressing the push button will short circuit the transmitter and receiver and allow enough current to flow to ring the bell at the other station. The other person has merely to lift up his receiver in order that the two parties may talk back and forth. When the receivers are on the hooks both bells are in circuit ready for either party to call up. CHAPTER X. MEDICAL COILS AND SHOCKING COILS. Medical coils and "shocking coils" are the same. The term "shocking coil" is usually applied to the low priced medical coils which most boys are familiar with for giving "shocks". [Illustration: FIG. 98.—Bobbin for Medical Coil.] A medical coil is nothing more or less than a small induction coil for raising the voltage of a battery high enough to produce a shock. The human body possesses so much resistance that the voltage of one or two cells of ordinary battery is not sufficient to pass enough current through to be felt except under exceptional conditions, as for instance when contact is made directly with a nerve through an open cut or a metal filling in a tooth. A small lathe will be of much assistance in making some of the parts and fittings for the medical coil, although it is not absolutely essential and the ingenious young experimenter can undoubtedly find some method of doing without it. It is not necessary to follow the dimensions given here exactly, and a very good coil can be built by adapting spools, binding posts, etc., which may already be on hand, although, of course, they should be used with some sense of proportion in mind so that nothing is carried to an extreme.