The block B is a small piece of hard-wood screwed fast to the side of the base to hold the set-screw and also the wire that comes from the outside of the upper coil. A small hole is made in the edge of the block and the wire passed in, so that the end rests in the screw-hole as shown by the dotted line. When the screw is placed in the hole and turned, it comes into contact with the wire and makes a connection. This block and its attachment is shown in Fig. 14 B. On the base, near the armature-block, a binding-post is made fast, and the current, passing in through the wire A in Fig. 9, goes through the coils and around to the screw B, then through the armature to the block, and out through the wire C. In its circuit the bolts are magnetized, and they draw the armature, but the instant they do so the loose spring-brass end is pulled away from the screw-point B and the circuit is broken, the bolts cease to be magnetized, and the armature flies back under the influence of the spring-brass neck at D. The loose brass end, on touching the screw-point, conducts the current through the coils again, with a continual vibrating action, so long as the electric current is passing in at A and out at C. The greater the volume of current the greater the number of vibrations, and to properly regulate the contact the set-screw B must be adjusted at the right point. Paste pieces of heavy paper over the heads of the bolts to overcome residual magnetism. A single electric bell is made the same as a buzzer, but continuing on from the end of the armature a wire or rod is mounted with a ball at the end which strikes the bell as the current causes the armature to vibrate. The bell-block may be made longer, and a bell from an old clock or a bicycle should be mounted at the proper place on a wooden dowel driven into the base. A screw passes through the hole at the middle of the bell and into the top of the dowel. The ball at the end of the rod may be made of brass with a hole in it, and a drop of solder will hold it in place. Or it may be made of wire wound round the end and soldered into a compact mass. A Large Induction-coil As has been said, the induction-coil is one of the mysterious phenomena of electrical science. While its practical value is known and recognized in all branches of voltaic electricity for use in transforming currents, its actual workings have never been clearly explained. The construction of a small induction-coil was explained in the description of a shocker or medical battery. For bigger equipments, wireless telegraphy and other uses, a large induction-coil will be necessary, and the following illustrations and descriptions should enable the young electrician to construct an apparatus that will be both simple and efficient in its working.