When the primary coil is finished off, cut three pieces of hard rubber four inches square, with a central hole just big enough that they may be slipped on over the primary coil to form divisions into which the secondary wire goes. These can be fixed equal distances apart by means of removable wooden blocks, which are taken off as each section is wound.
Secondary Coil.
The secondary coil consists of about 2-1/2 pounds No. 36 B. & S. gauge silk or cotton-covered magnet wire, wound evenly in layers in the sections on the primary coil. Before any wire goes into a section, it must be seen that the division fits tight to the primary coil. It will be best to pour around the coil some melted paraffin wax so as to form an insulating ring, and prevent any possibility of the spark creeping under the section division into the next. The actual operation of winding presents no difficulty other than those of keeping the wire from damage and getting as even layers as possible. If each layer is separated from its neighbor by a strip of paraffined paper, it makes even winding easier, and better insulation. As to the insulating of the secondary coil, it can be done in a variety of ways. The coil can be soaked in molten paraffin until saturated, or the wire can be made to pass through a dish of molten paraffin while on its way from the wire reel to the coil. In the latter case the wire must be guided by means of glass rollers, as the wax would harden rapidly if touched by the fingers. In connecting up the sections, the similar ends are to be joined; that is, the inside ends to inside ends, and outside ends to outside ends, as per diagram (Fig. 45). This will bring two outside ends free for attachment to binding posts. Fig. 46 shows direction of winding and connecting the two middle coils, A C being the inside layers next to primary and B D the outside layers.
Fig. 45.
An outside coat of paraffin wax is now given to the coil and a wrapping of waxed paper laid on. Then, if desired, a cover of sheet-rubber or a layer of cloth can be put on over all, to finish the job.
Base, Key, and Condenser.
Fig. 46.
The base for a Ruhmkorff coil generally resembles an oblong shallow box. The coil is mounted on the lid, and the condenser inside the box, the connections being made on the lower side of the lid. It is preferable, except for appearance’ sake, to make all connections outside the box, but this is left to the worker’s choice.