The primary should be wound with a single layer of No. 22 single-silk-covered magnet wire. The secondary is wound with No. 29 single-silk.
The head supporting the secondary is smaller than that used for the same purpose on the primary. The round boss to which the tube is fastened, however, is much thicker.
The secondary slides on a "guide-rod" supported at one end by passing through the primary head and at the other by a brass upright. The upright may also be made of wood.
If the secondary is "offset," that is, placed out of center slightly to one side, it will leave room so that the secondary slider will possibly pass inside of the primary without striking.
Both the primary and the secondary must be fitted with "sliders" to make contact with the various turns of wire.
The method of constructing a slider has already been described.
The ends of the slider-rods are bent at right angles and fastened to the coil heads by two small screws passing through holes bored near the ends. A small narrow path must be scraped in the insulation under each so that the slider will make contact with each turn. The secondary head may be provided with a small electrose handle to facilitate sliding it back and forth.
Two binding-posts are mounted on each of the coil heads.
One post on each is connected to the end of the coil farthest from the head, and the other posts are each connected to the slider-rods.
Figure 220 shows how to connect the loose coupler in the receiving set.