Harper's Electricity Book for Boys

by Joseph H. Adams

Make a hollow wooden base for the induction-coil to rest on. It should be a trifle longer than the length of the coil and about seven inches wide. This may be made from wood half an inch thick. The base should be two inches high, so that it will be easy and convenient to make wire connections under it. Mount the induction-coil on the base and make it fast with screws, arranging it so that the binding-posts are on the side rather than at the top of the coil, as shown in [Fig. 13].

Cut a thin board and mount it across the top of the induction-coil on two short blocks, and to this attach two double-pole binding-posts (P P). The fine wires from the induction-coil are made fast to the foot of each post, and from the posts the aerial wire (A W) and ground wire (G W) lead out.

Fasten two binding-posts at the forward corners of the base, and to them make connection-wires fast to the heavy or primary wires of the coil. Wires B and C lead out from these posts to the battery and key, and to complete this part of the sending, or transmitting apparatus it will be necessary to have two terminal rods and balls attached to the top of the binding-posts (P P). This part of the apparatus is generally called the oscillator, and the rods are balanced on the posts, so that they can be moved in order to increase or diminish the space (S G), or spark-gap, between the brass balls.

When, after experiment, the proper space has been determined, the set screw at the top of the posts will hold the terminal rods securely in place.

Obtain a piece of brass, copper, or German-silver rod three-sixteenths of an inch in diameter. Now cut two short rods, each six inches long, and two inches from one end flatten the rods with a hammer, as shown at A in [Fig. 14]. Flatten the rod in two places at the other end, as shown at B B in [Fig. 14]; then bore holes through the flattened parts (A), so that the binding-screws at the top of the posts (P P) will pass through them.

Obtain two brass balls from one to one inch and a half in diameter. If they are solid or cast brass they may be attached to the ends of the terminal rods by threading, so that it will be easy to remove them. If the balls are of spun sheet-metal it will be necessary to solder them fast to the ends of the rods, and, when polishing the balls, the rods will have to be removed from the binding-posts. It is imperative that the balls should be kept polished and in bright condition at all times, to facilitate the action of the impulsive sparks.

To counterbalance these balls there should be handles at the long ends of the rods. These handles may be of wood, or made of composition molded directly on the rods. A good composition that can be easily made and molded is composed of eight parts plaster of Paris and two parts of dextrin made into a thick paste with water. The dextrin may be purchased at a paint-store, and is the color of light-brown sugar. Mix the dry plaster and dextrin together, so that they are homogeneous; then add water to make the pasty mass. Use an old table-knife to apply the wet composition to the bars. The flattened parts will help to hold the mass in place until it sets. It is best to make two mixtures of the paste and put one on first, leaving it rough on the surface, so that the last coat will stick to it. When the last coat is nearly dry it may be rubbed smooth with the fingers and a little water, or allowed to dry hard, and then smoothed down with an old file and sand-paper.

If solid brass balls are used for the terminals the composition handles may be made heavier; but in any event the proper amount of composition should be used, so that when the rod is balanced on a nail or piece of wire passed through the hole it will not tip down at one end or the other, but will remain in a horizontal position.