119. Washers or coil ends are used on the bolt magnets so that considerable wire can be wound on closely and evenly. These are made out of thick pasteboard, which cuts smoother if it has been soaked in melted paraffine. Unless you know how, you will find it a hard job to make the hole in the exact center of the washer. The method of easily making washers is illustrated in Fig. 63.
First place a spool (the end of which is ⅞ or 1 in. in diameter) upon the table, and lay the pasteboard upon this. Push a large round nail through the pasteboard into the hole in the spool. The nail should be nearly as large as the hole. Use the large nail as a handle, and with the shears cut around the edge of the spool end. Cut the washer as round as possible, and be careful not to cut into the spool.
The holes in the washers will be a little smaller than the 5⁄16 bolt. This will make the washers hold tightly to the bolt when you force them on. Fig. 64 shows the bolt-core, with the washers in place. If you cannot get a large nail, a lead-pencil, or sharpened dowel, will do to force through the pasteboard.
120. Insulation of Cores. While the covering on the wire would probably be all that is necessary to thoroughly insulate the coil from the core, it is better to wind a layer or two of paraffine paper around the bolt (Fig. 65) before winding.
121. The Coils of wire to be used upon the bolt-cores should be put on with the winder ([App. 93]). For all ordinary purposes No. 24 or 25 single or double cotton covered copper wire will do. It is better to put on an even number of layers. The winding (See Fig. 70) begins at the nut-end of the bolt, and by using 6 or 8 layers of wire, instead of 5 or 7, both coil ends will be at the same end of the bolt.
| Fig. 64. | Fig. 65. | Fig. 65½. |
122. Method of Winding the Coils. The winders used for bolt magnets are described in [App. 91], etc. We shall suppose that the washer, W, Fig. 70, and the insulation, I, are upon the bolt before screwing it into the winder-nut, W N. Make a pinhole, P H, in the right-hand washer, as near the bolt-nut, B N, as possible. Stick about 6 in. of the wire through P H, and wind this end around W N, as shown, to hold the wire. The supply of wire should be upon a spool slipped onto some stationary rod ([App. 23]), so that you can give your entire attention to winding. Begin to turn the winder slowly at first. Turn the handle towards you when it is at the bottom, as in Fig. 70; that is, if you look at it from the side, turn the handle clockwise. Let the wire slip through your left hand as the turns are made, and guide it so that the turns will be close together. If they go on crooked, unwind at once, then rewind properly. You can guide the wire best by holding your left hand about 8 or 10 inches from the bolt. As soon as you reach the left side or head end of the bolt, feed the wire towards the right. If at any time the layers become rough on account of one turn slipping down between turns of the previous layer, fasten a piece of paraffine paper around the coil as soon as the imperfect layer is completed. Wind on 8 layers, and count the number of turns in one or two of them, so that you can tell about how many turns in all you have around the core. Make a "half-hitch" ([see § 110]) with the wire when the last layer is finished, to keep it from unwinding, and leave a 6 in. end.
The coil should be protected by fastening around it a piece of dark-colored stiff paper. Paraffine paper is good for this purpose. With a little practice you will be able to rapidly and neatly wind on the wire. The winder-nut, W N, must hold the bolt solidly to keep it from wobbling.
123. We shall call the starting end of the wire which passes through P H, the inside end, and the end of the last layer the outside end. This can pass out between the washer and the paper covering.