The connections are shown in the diagram (Fig. 39). Suppose the current to enter at the binding post _a_, pass around the magnets _b_ and then to the post _c_. The armature _d_ normally rests against the post _c_ and the current finds its way along this to the post _e_ and thence back to the battery. But as soon as the current passes, _b_ becomes a magnet and pulls the armature _d_ away from the post _c_, thus breaking the circuit, when _b_ ceases to be a magnet and a spring pushes the armature _d_ back against the post _c_ to repeat the operation. The armature _d_ carries a hammer which strikes the gong _f_. If the wire, which is usually connected with the binding post _e_, is connected with the post _c_, the "clatter" bell is changed to a "single-stroke" bell, and if the gong and hammer are removed the "bell" is changed to a "buzzer." [Illustration: Fig. 40] In the case of the buzzer, by changing the length of the armature or by weighting it, we may change the time of its vibrations and its tone. The connections between battery push button and bell form a complete circuit. In Fig. 40 _B_ represents a battery, usually of dry cells, _B'_ represents the bell, and _P_ represents the push button. The electric circuit is "open," (that is, there is a break in the conductor) at _P_ until some one "pushes the button," that is, simply pushes against a spring so as to cause a piece of metal to bridge the gap in the conductor. Then we say the circuit is "closed." [Illustration: Fig. 41] [Illustration: Fig. 42] Push button devices and switches are innumerable. In every case they are simply devices for pushing one piece of metal against another and completing the circuit for an electric current. Every one should unscrew and examine a few of them, both for the pleasure of seeing how they work and to learn how to make them work when they sometimes fail. Not only in bells but in all other instruments where electro-magnets are used, the magnets are placed in pairs, fastened together upon an iron base. They are wound so that the free ends are made opposite poles by the electric current. Like a horseshoe magnet, they form one magnet. The two poles thus placed are mutually helpful and each is stronger than it would be if separated from the other. [Illustration: Fig. 43]