ELECTRIC BELLS.

The electric bell is a bell rung by electricity.

Generally it is worked by a current exciting an electromagnet attracting or releasing an armature which is attached

ELECTRIC BELLS.

The arrangements of the instrument are shown in Fig. 276, in which E is the electro-magnet and H the hammer. A battery consisting of one or two Leclanché cells placed at some convenient point of the circuit, provides a current when required. By touching the "push" P, the circuit is completed, and a current flows along the line and round the coils of the electro-magnet, which forthwith attracts a small piece of soft iron attached to the lever, which terminates in the hammer H. The lever is itself included in the cireuit, the current entering it above and quitting it at C by a contactbreaker, consisting of a spring tipped with platinum resting against the platinum tip of a screw, from which a return wire passes back to the zinc pole of the battery. As soon as the lever is attracted forward the circuit is broken at C by the spring moving away from contact with the screw; hence the current stops, and the electro-magnet ceases to attract the armature. The lever and hammer therefore fall back again establishing contact at C, whereupon the hammer is once more attracted forward, and so on. The push P is shown in section on the right of Fig. 277. It usually consists of a cylindrical knob of ivory or porcelain capable of moving loosely through a hole in a circular support of porcelain or wood, and which, when pressed, forces a platinum-tipped spring against a metal pin, and so makes electrical contact between the two parts of the interrupted circuit.

The bell may be worked by a distant switch or press button ringing once for each movement of the distant switch or it may be of the

ELECTRIC BELLS.

Vibrating Bell Type.-When the current is turned on in this system it attracts the armature. As this moves towards the poles of the magnet it breaks the circuit by drawing the "contact spring" away from the "contact point."

This opens the circuit as before explained and causes the continual ringing.

The possible defects of electric bells may be classed uuder four heads, viz.: Ist, bad contacts; 2d, bad adjustment of the parts; 3rd, defective insulation; 4th, warpage or shrinkage of base. Many operators are content with simply turning the

ELECTRIC BELLS.

terminal wires round the base of the binding-screws. Unless the binding-screws are firmly held down on to the wires by means of a back nut a great loss is sure to occur at these points, as the wires may have been put on with sweaty hands, when a film of oxide soon forms, which greatly lowers the conductivity of the junction. Again, at the junction points of the wires with the contact angle brass and contact pillar, some workmen solder the junctions, using “killed spirits” as a flux.

If solder be used at any parts, let resin be used as a flux. Even if any excess of resin remain on the work, it does no harm and does not destroy the insulation of any of the other portions. Another point where bad contact may arise is at the plantinum contacts. Platinum is a metal which does not rust easily, even under the influence of the electric spark given at the point of contact. Therefore it is preferred to every other metal (except, perhaps, iridium) for contact breakers.

As to bad adjustment; it is evident that the magnets and the armature must stand at a certain distance apart to give the best effects with a given battery power. The distance varies from in. in the very smallest, to in. in large bells. Sometimes the armature adheres to the poles of the electromagnet; this is due to residual magnetism, and points to hard or unannealed iron in the cores or armature. As a make-shift, this defect may be partially remedied by passing a thin piece of paper over that surface of the armature which faces the poles of the electro-magnets.