Make the magnets from one-quarter-inch stove-bolts, and cut them off so that they will form a core about two and one-quarter inches long. Fit each of the cores with two fiber heads to hold the wire in place. Insulate the legs with paper and wind each with about fifty layers of No. 30 B. & S. gauge single-cotton-covered magnet wire. The winding space between the magnet’s heads should be one and one-eighth inches. The upper ends of the magnet cores should be allowed to project about one-quarter of an inch beyond the fiber head. The end of the core is filed flat, as shown in the illustration. The magnets are mounted upon an iron yoke, three-sixteenths of an inch thick. The holes in the yoke should be spaced so that there is a distance of one and one-half inches between the centers of the magnet cores. The armature of the relay is mounted on a small steel shaft with sharp points at each end. The exact shape of the armature may be best understood from the illustrations. The lower end of the shaft rests in a small cone-shaped depression made by driving a center punch into the yoke half-way between the two magnets. The top bearing is a strip of brass projecting from a wooden support. The end of the shaft rests in a depression similar to that in the yoke. The contact lever is made of brass and forced on the shaft below the armature. It swings between a small brass clip fastened to one side of the support and a little screw held in a similar clip on the opposite side. The contact clip is made of spring brass about No. 22 gauge in thickness. It may be adjusted by a screw passing through the support. The armature may be controlled in its movement so that the latter will be very slight by adjusting the screws. There should not be any friction in the bearings and the armature should move with perfect freedom. The armature should approach the ends of the magnet cores until a space about the thickness of heavy paper separates them and should not touch them. [Illustration: Fig. 139.—The Completed Relay.] The spring is made of fine brass wire. It is fastened to the armature shaft, and the screw mounted on the wooden support with a piece of silk thread. The thread is passed around the shaft once or twice so that the tension of the spring will cause the armature to move away from the pole pieces just as soon as the current flowing through the magnets ceases. [Illustration: Fig. 140.—A Diagram showing how to connect a Relay, Sounder, and Key. Closing the Key will operate the Relay. The Relay will then operate the Sounder in turn.] The tension of the spring may be adjusted by turning the screw with a screw-driver. If the armature tends to stick to the magnet poles fasten a small piece of paper to the poles with some shellac. The terminals of the magnets are connected to two binding-posts marked _A_ and _B_. The binding-posts marked _C_ and _D_ are connected respectively to the contact clip and the brass bearing on the top of the wooden support.