Having now considered electricity in three of its forms—static, current, and rotatory—we will pass to some of its applications.
THE ELECTRIC BELL.
A fit device to begin with is the Electric Bell, which has so largely replaced wire-pulled bells. These last cause a great deal of trouble sometimes, since if a wire snaps it may be necessary to take up carpets and floor-boards to put things right. Their installation is not simple, for at every corner must be put a crank to alter the direction of the pull, and the cranks mean increased friction. But when electric wires have once been properly installed, there should be no need for touching them for an indefinite period. They can be taken round as many corners as you wish without losing any of their conductivity, and be placed wherever is most convenient for examination. One bell may serve a large number of rooms if an indicator be used to show where the call was made from, by a card appearing in one of a number of small windows. Before answering a call, the attendant presses in a button to return the card to its normal position.
In Fig. 52 we have a diagrammatic view of an electric bell and current. When the bell-push is pressed in, current flows from the battery to terminal T1, round the electro-magnet M, through the pillar P and flat steel springs S and B, through the platinum-pointed screw, and back to the battery through the push. The circulation of current magnetizes M, which attracts the iron armature A attached to the spring S, and draws the hammer H towards the gong. Just before the stroke occurs, the spring B leaves the tip of the screw, and the circuit is broken, so that the magnet no longer attracts. H is carried by its momentum against the gong, and is withdrawn by the spring, until B once more makes contact, and the magnet is re-excited. The hammer vibrations recur many times a second as long as the push is pressed in.
Fig. 52.—Sketch of an electric-bell circuit.
The electric bell is used for so many purposes that they cannot all be noted. It plays an especially important part in telephonic installations to draw the attention of the subscribers, forms an item in automatic fire and burglar alarms, and is a necessary adjunct of railway signalling cabins.
THE INDUCTION OR RUHMKORFF COIL.