No. 24 B. S. gauge copper wire may be used for winding both the primary and secondary. It is also the proper size to use on the double slide tuner. The best method is to use bare wire, wound with a thread so that a thread is interposed between adjacent turns of the winding. Give the whole winding one or two coats of thick shellac and allow it to harden. Then use a strip of sandpaper to remove the shellac in a long narrow path immediately below the sliders so that they may make contact with the wire.
Fig. 130. Slider for Loose Coupler.
Some may prefer to make a loosely coupled tuner in which the inductance of both coils is adjustable by means of a sliding contact. In such a case the slider on the secondary coil must be constructed as illustrated in Fig. 130. The contact is long and narrow so that it can touch the innermost turns, when placed within the primary. By slipping the slider off the end of the rod and reversing it, the contact can be made to touch the turns next to the head. The square brass rod is set in a notch cut in the coil head so that the rod is flush with the top.
When tuning a receiving transformer, place both variable condensers in a halfway position and adjust the sliding contacts, first on the primary and then on the secondary, until the signals are the loudest. Then adjust the condensers.
Fig. 131. Loosely Coupled Tuning Circuits.
To cut out an undesirable station, vary the coupling between the two coils by sliding the secondary away from the primary. When several turns on the secondary seem to give the same results also vary the coupling.
Fig. 132. Combination Loosely and Closely Coupled Tuner.