The magnets are clamped down to the bell-base by means of a hard-wood strip having a screw passed through it between the magnets into the base. [Illustration: Fig. 119.—Details of the Magnet Spools, and Yoke for an Electric Bell.] The armature of the bell is shown in Figure 120. It is made of a piece of iron having a steel spring riveted to it, as illustrated. The armature is fastened to a small block mounted on the lower left-hand corner of the base. [Illustration: Fig. 120.—Details of the Armature, and Contact Screw.] A second block with a contact-point made from an ordinary brass screw by filing the end into the shape shown in the illustration, is mounted on the base so that it is opposite the end of the contact-spring fastened to the armature. The gong may be secured from an old bell or alarm clock. It is mounted on the upper part of the base in such a position that the hammer will strike it on its lower edge. The instrument is connected as shown in Figure 121. One terminal of the magnets is connected to the contact-screw. The other end is connected to the binding-post. A second binding-post is connected to the armature. [Illustration: Fig. 121.—The Completed Bell.] The armature spring should be bent so that the armature is pushed over against the contact. If a battery is connected to the bell, the electromagnets will pull the armature and cause the hammer to strike the gong. As soon as the armature has moved a short distance, the spring will move away from the contact and break the circuit. The magnets cease pulling as soon as the current is cut off and the armature spring then causes the armature to move back and touch the contact. As soon as the contact is made, the armature is drawn in again and the process is repeated. [Illustration: Fig. 122.—Diagram showing how to connect a Bell, Battery, and Push-Button.] A little experimenting with the bell will soon enable one to find its best adjustment. Figure 122 shows how to connect a bell to a battery and a push-button. A push-button is simply a small switch which closes the circuit when pressed. Do not make the armature spring too weak, or the hammer will move very slowly and with very little life. Each time that the armature moves toward the magnets, it should barely touch the iron cores before the ball strikes the bell. After you get the bell in good working order, it is well to make a small box to serve as a cover for the working parts of the instrument, leaving only the gong exposed. [Illustration: Fig. 123.—Two Simple Push-Buttons.] [Illustration: Fig. 124.—Diagram showing how to arrange a Bell System of Return Signals.]