The primary coil should be made of 3 layers of wire, which should be coarser than that used for the secondary coil. For our purposes it is best not to use a wire coarser than No. 20, and not finer than No. 24.
Use No. 24 insulated copper wire if you are going to connect ordinary batteries with it. A bichromate cell ([App. 4]) is best. Put about 6 in. ([see § 109]) of wire through H, and with [App. 93] wind on 3 layers of say No. 24 wire. There being an odd number of layers, the winding will stop at the head end of the bolt, where a half hitch ([see § 110]) should be taken before passing the wire through the hole, J. Cut the wire 6 in. from the hole. Write down the number of turns of wire to each layer and the total number of turns. You now have a 3–layer coil, and a current passed through this will magnetize the bolt; you have—so far—merely an electro-magnet. Cover the primary coil with 2 layers of paraffined paper, K (Fig. 74), and put some paraffine between the edges of K and the washers, so that the wire of the secondary coil cannot possibly come in contact with that already wound on.
138. The Secondary Coil should be made of a large number of turns of fine wire. Do not use anything coarser than No. 30. This is a good size, as finer wire is very easily broken by unskilled hands. For the size of bolt mentioned put on 13 layers. There will be about 100 turns to each layer, making a total of about 1,300 turns of No. 30 wire. Write down the total number of turns in your coil. To start the secondary coil, make a pinhole, L, just outside of the insulation, K, of the primary coil. Put 6 in. of wire through this, wind the end around the nut ([App. 93], Fig. 70), and wind on as evenly as possible 13 layers. If the layers become rough, it is well to put a band of paper around after each 3 or 4. When you have finished take a half hitch ([§ 110]), and leave a 6-in. length free. Cover the secondary coil with strong paper. This coil may be used on any of the forms of shockers given.
Fig. 75.
139. Induction Coil. Fig. 75. The base is made of a piece of board, 7 × 5 × ⅞ 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 ½ or ¾ 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 ¼ 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.