Three layers of the coarse wire should be wound on first, for
The Primary-coil. Pierce a hole through one cardboard end, stick the wire through it, and allow about 5 inches to project upon the outside; then commence winding the wire upon the core, placing each turn close to the preceding turn. When the opposite end of the bolt has been reached, wind back to the starting point, then work back to the other end again. There will be in the neighborhood of 175 turns in the three layers. Cut off the wire so there will be a 5-inch projection, and stick the projecting end through a hole in the cardboard end. This completes the primary-coil ([Fig. 189]).
Before winding the small wire on top of the primary-coil, to form
The Secondary-Coil, wrap the primary-coil with a layer of bicycle tape, or glue several layers of paper around the coil. Then wind on the small wire as you did the coarser wire, being very careful to get it on evenly and smoothly. Wind eleven layers on the coil, and run the end of the eleventh layer out through the cardboard end ([Fig. 190]). There should be about 100 turns of this wire to the layer, or 1100 turns in all.
A crank arrangement can be rigged up to make the winding easier, but with patience, and by doing the work slowly, the wire can be wound almost as well by hand. It is difficult to keep track of each preceding turn, while winding, because of the fineness of the wire, and on this account it is a good scheme to coat each layer with bluing after it has been wound on, so that each turn of the following layer will show plainly against the stained layer beneath it. [Fig. 190] shows the complete induction-coil.
Cut a base block 5 inches wide and 7 inches long, bevel the top edges to give it a trim appearance, and mount the induction-coil to one side of the center ([Fig. 187]), strapping it in place by means of two tin straps similar to that shown in [Fig. 191], cut from a tin can.
The projecting ends of the primary-coil connect with the battery, while the two ends of the secondary-coil connect with the handles. Make three binding-post plates out of folded pieces of tin, similar to plates B and C, in [Fig. 182]. Tack two of these to the end of the base and connect the secondary-coil wires to them ([Fig. 187]), and tack the third near one end of the induction-coil and connect one primary-coil wire to it ([Fig. 187]).
For the Handles take two pieces of broom-handle 3½ inches long, and cover each with a piece of tin ([Fig. 192]). The pattern for the tin covering ([Fig. 193]) shows how tabs are prepared on the ends and holes punched through them for connecting with the induction-coil. The connecting wires should be 5 or 6 feet long. Flexible wire is better than bell-wire for these, because it is more easily handled in passing the handles around. Tack the tin covering to the pieces of broom-handle.