Harper's Electricity Book for Boys

by Joseph H. Adams

A Double Electric Bell

For loud ringing, and to get the benefit of both the forward and backward stroke of the knocker, a double bell, similar to the one shown in [Fig. 4], may be constructed upon the general principle of the single-stroke buzzer already described ([chapter iv.]).

Two soft iron cores are made, as described for the other bells, but instead of being yoked together with iron, so that the three parts will form a horseshoe magnet, the yoke is of brass or copper. Each core will then be an independent magnet.

The spools are wound with No. 22 insulated wire and the ends left free, so that the coils are not connected together. If the drawing is examined closely you will see that the armature swings on a pivot at the base of the knocker-bar. When the bell is not in action the knocker might rest naturally against one bell or the other; or it might stand in the centre and not touch a contact-point, were it not for the small spring which draws it to the left. Directly the current is run through the coils it alternately magnetizes first one and then the other. This action is due to the making and breaking of the circuit by the spring on the armature. It first comes into contact with one point, and then is drawn away from it to come into contact with the other. [Fig. 4] shows the knocker-bar at rest between both bells and the armature unattracted by either magnet. This position is purposely given so as to indicate the balance of the armature and the spaces between it and the cores and also the contact-points above it.

The small, light wire spring shown in the drawing draws the knocker to one side; therefore, when at rest, one circuit is closed. Otherwise the bell would not act when the current is run through the parts—in fact, the current could not run through at all, if one or the other contact were not made.

The magnets are held fast to a base with a long screw and a small plate of wood laid across them as shown in [Fig. 4]. The armature is a piece of soft iron one-eighth of an inch thick, half an inch wide, and three inches long. This has a spring-brass piece attached to it as shown at A A in [Fig. 5]. Small holes are bored through the strip and the iron, and escutcheon pins are passed through and riveted. A small hole is made down through the middle of the iron plate and a pin is driven into it, so that a quarter of an inch projects at both sides.

Another hole is made through the side of the plate for the knocker-bar. Then the armature is set in place so that there is a space of one-eighth of an inch between it and the magnet ends. The armature is held in place at the top by a bent metal strip (B B in [Fig. 5]). This is screwed fast to the base and the bottom is countersunk into the wood.

Two contact-points (C C in [Fig. 5]) are arranged so that when a magnet draws the armature down it brings the opposite end of the armature spring into contact with a point.

The wiring is at the under side of the base and is shown in [Fig. 6]. The current enters binding-post A, and passes around coil and magnet No. 1 by entering at B and leaving at C; from thence to D, entering the armature spring at E, when the small spring has drawn the knocker-bar over to the left. The current passes along the armature and out at F; then along to binding-post G, and so on around through battery K and push-button L, thus completing the circuit. Directly that this is done the magnet draws the spring end of the armature away from contact-point D and up against contact-point J, so that the circuit is broken through coil No. 1 and is sent through coil No. 2. This immediately magnetizes core No. 2 and draws the armature down to it, breaking its contact with J and re-establishing it with D. The rapid alternate making and breaking of the circuit, and the rapid and strong motion of the armature in its seesaw action, causes the knocker to rap the bells soundly each time it travels from right to left and back again.

Two bells of similar size, or two drums or wooden boxes, may be employed for this double striker, and the more current that is run through the coils the more power and a corresponding increase of noise.