First, draw the circle out with a compass to the diameter shown and then divide off the outside into an unequal number of divisions as shown. Draw a light pencil line through each of these marks to the centre of the circle. Now with your scissors cut out the disc, after which you cut the slots as shown.
The slots should be about one-quarter of an inch in width and of the depth shown in the drawing. Two such discs should be made and, when all cut out, should be given several coats of shellac to add stiffness and to improve the insulating qualities.
Now at your hardware dealer’s buy one-quarter pound of No. 24 double, cotton-covered wire and proceed to wind the coils in the manner shown. Keep the windings even and avoid all joints throughout the length of winding.
When you have finished, mount the coils as shown in the drawing. Make sure that the windings on both coils run in the same direction. If you fail to do this, the set will not work.
For the detector, it is better to purchase a good make of galena detector at any radio supply store. If you are handy with tools, however, you can buy the galena and make your own detector. It will work with more or less satisfaction.
Your next need will be the condenser. The condenser consists of a series of aluminum plates, some of which are movable and the rest stationary.
Buy a small variable condenser. Its function is to tune the secondary circuit, which is accomplished simply by turning the knob. Such a condenser could not be made without the use of a good set of tools, and the author strongly advises it be bought instead of made at home in order to avoid trouble. The aluminum plates are spaced very closely and great care should be taken to avoid bending them, as they must not touch each other.
The aerial for this set should be about 60 to 100 feet in length and as high and clear of surrounding objects as possible. A simple porcelain cleat at either end, as shown in the drawing, will serve to insulate it sufficiently.
Your ground connection can be made best by wiring to the cold water pipe, although wiring to a steam or gas pipe will do almost as well.
You are now prepared to mount the various instruments in their proper locations. For your table instruments, get a good pine board about seven-eighths of an inch thick. Buy four binding posts and use one for the aerial wire, one for the ground wire, and two for the phones or head set.
To operate the set, first bring the hinged coil of wire close up to the fixed coil and adjust the detector until you can hear in your receivers the loudest click caused by the turning on and off of the key to a nearby electric light. If no light is available, a buzzer and dry battery should be used. When the detector is properly adjusted you will be able to hear the buzz quite distinctly in the head phones if the buzzer is not too far away.