139. Induction Coil. Fig. 75. The base is made of a piece of board, 7 × 5 × 7/8 in. The locations of the different parts are shown in the figure. The coil is explained in detail in App. 96. It is fastened to the base by a thin copper strip, 4, which is bent over the coil and held down by screws, 3. If you haven't any copper you can use a narrow strip of tin. Do not use a wide piece of tin or iron. The coil may be held down firmly by strong twine placed around each end of it. The twine should pass through holes in the base, and be tied on the underside of the base. The binding-posts are like App. 46.
140. The Current Interrupter consists of a tin or copper strip, R, 6 in. long and 1/2 or 3/4 in. wide. At one end of R is a screw, S, which is used as a binding-post for the outside end, B, of the primary coil. (See § 137.) Along the center line of the strip, R, are driven 1-in. wire nails, Q. These are placed 1/4 in. apart, and they should go into the wood enough only to make them solid. (See Fig. 81.) Do not drive them in so far that they will split the base. A stout wire, P, fastened at one end only completes the interrupter.
141. The Connections. The binding-posts, W and X, should be connected with the wires leading from a battery. Use the bichromate batteries of App. 3 or 4. A dry battery will do. If the current enters at X, it will pass around the primary coil (§ 137) and out through B into R. It can go no farther until the free end of P is made to touch R, or one of the nails, Q, when the circuit will be closed. The current will fly around and around through the battery, primary coil, and interrupter as long as the end of P touches a nail. The battery current does not get into the secondary coil at all. You can see, then, that the primary circuit, that is, the one passing through the coarse wire, will be rapidly opened and closed by bumping the free end of P along upon the row of nails.
The wires, C and D, coming from the secondary coil (§ 138) are in connection with Y and Z, to which are connected the wires leading from the handles (App. 101) held by the person receiving the shock.
142. To use the coil, arrange as explained. Let your friend hold the handles (App. 101) while you scrape the end of P back and forth along the row of nails. For those who cannot stand much of a shock, use a regulator (App. 103).