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. [Illustration: FIG. 13] [Illustration: FIG. 14]