Slip the spool on the winder (Fig. 7) and wind on three layers of No. 24 cotton-insulated copper wire, taking care to wrap the coils evenly and close. Bring six inches of the ends out at either end of the spool through small holes pierced in the flanges; then wrap several thicknesses of brown paper around the coil. A current passing around this three-layer coil will magnetize the bolt. This is the primary coil and the one through which the battery current will pass. A secondary coil is now made over the primary one with eleven or thirteen layers of No. 30 insulated copper wire. It will take some time to carefully put on these layers, and when doing so mark down each layer so as to keep an accurate count, for there must be the right number of layers to make the coil act properly. No. 30 wire is quite fine, and if the layers are not inclined to lie smooth, make a wrap or two of brown paper between each three layers. Bring six inches of each end of the wire out from the flanges of the spool, and to protect the outer coil wrap paper about the coils and attach it fast with thread or paraffine. Slip the bolt through the hole and screw the nut on the threaded end. Cut out a wooden block four inches long, three inches wide, and three-quarters of an inch thick, and with two thin metal straps and screws attach the coil to the middle of the block, as shown in Fig. 8. Make four binding-posts and screw them fast at the corners, and to A and B of Fig. 8 attach the ends of the heavy wire from the primary coil, and to C and D of Fig. 8 the ends of the fine wire from the secondary coils. The induction-coil is now ready for any use to which it may be put, and by mounting it on the block with the delicate wire ends attached to the binding-posts, it is in less danger of damage than if the wire ends were left loose for rough-and-ready connections. In order to get a shock from this coil it will be necessary to have a pair of handles and a current interrupter. The handles may be made from two pieces of tin rolled into the form of cylinders to which wires are soldered. Or, better yet, use pieces of thin brass tubing an inch in diameter. The buzzer shown in Fig. 9 may be employed for a current interrupter, and a bichromate battery will furnish the current. In order to make the connections, the wires from the handles are attached to the binding-posts C and D in Fig. 8--that is, to the wires of the secondary coil. One spool of the battery is connected with A of Fig. 8 and the other with A of Fig. 9. A wire connects C of Fig. 9 with B of Fig. 8, and the circuit is closed. The buzzer now begins to vibrate, and any one holding the handles will receive a shock the intensity of which depends on the strength of the batteries. A switch should be introduced somewhere in the circuit, so that it may be opened or closed at will; a good place for it is between a battery-pole and the binding-post A in Fig. 8.