Fig. 31.
§ 43. Having thus constructed an efficient electric bell we may proceed to study its action and notice some of the defects to which it may be subject. In the first place, if we connect up the bell with the battery as shown in [Fig. 17], viz., the left-hand binding-screw with the wire proceeding from the carbon of the Leclanché, and the right-hand screw with the wire from the zinc, then, if the platinum tipped screw touches the platinum speck, at the back of the contact spring, a current of electricity flows from the left-hand binding-screw all round the coils of the electro-magnets, passes along the contact spring and platinum speck, thence to the platinum tipped screw along the short length of wire to the right-hand binding-screw, whence it returns to the zinc element of the battery, thus completing the circuit. The current, in thus passing around the electro-magnet cores, converts them, pro tem., into a powerful magnet (see [§ 13]); con
sequently, the armature, with its contact spring and hammer, is pulled towards the electro-magnets and at the same time gives a blow to the bell. Now, if instead of having the platinum speck attached to a flexible spring, it had been attached bodily to the rigid iron armature, directly the electro-magnets felt the influence of the current, the platinum speck would have also been pulled out of contact with the platinum screw, therefore the electro-magnet cores would have immediately lost their magnetism (see [§ 13], last five lines). This would have been disadvantageous, for two reasons: 1st, because the stroke of the hammer would have been very short, and consequently the ring of the bell very weak; and, 2nd, because, as even the softest iron requires some appreciable time for the electric current to flow round it to magnetise it to its full capacity, it would need a much greater battery power to produce a given stroke, if the contact were so very short. The use of an elastic contact spring is, therefore, just to lengthen the time of contact. But the electro-magnets, even when the flexible spring is used, do actually pull the platinum speck out of contact with the platinum screw. When this takes place, the circuit is broken, and no more current can flow round the electro-magnets, the spring reasserts its power, and the contact is again made between the contact screw and contact spring, to be again rapidly broken, each break and make contact being accompanied by a correspondingly rapid vibration of the armature, with its attendant clapper, which thus sets up that characteristic rapid ringing which has earned for these
bells the name of trembling, chattering, or vibrating bells.
§ 44. From a careful consideration of the last two sections it will be evident that the possible defects of electric bells may be classed under four heads: viz., 1st, Bad contacts; 2nd, Bad adjustment of the parts; 3rd, Defective insulation; 4th, Warpage or shrinkage of base. We will consider these in the above order. Firstly, then, as to bad contacts. Many operators are content with simply turning the 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. A soldered contact is certainly the best, electrically speaking, but "killed spirits," or chloride of zinc, should never be used as a flux in any apparatus or at any point that cannot be washed in abundance of water, as chloride of zinc is very deliquescent (runs to water), rottens the wire, and spoils the insulation of the adjacent parts. 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 platinum 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. Platinum is an expensive metal, the retail price being about 30s. an ounce, and as it is nearly twice as heavy as lead (Lead 11. Platinum 21·5) very little goes to an ounce. For cheap bells, therefore, there is a great temptation to use some other white metal, such as silver, german silver, platinoid, etc.
The tip of the platinum screw may be tested for its being veritably platinum in the following mode: Touch the tip with the stopper of a bottle containing aquafortis, so as to leave a tiny drop on the extreme point of the suspected platinum. If it boils up green, or turns black, it is not platinum; if it remains unaltered, it may be silver or platinum. After it has stood on the tip for a minute, draw it along a piece of white paper, so as to produce a streak of the acid. Expose the paper for a few minutes to sunlight. If the streak turns violet or pinky violet, the metal is silver; if the paper simply shows a slightly yellowish streak, the metal is platinum. The tip of the platinum screw must be carefully dried and cleaned after this trial before being replaced.
Secondly, 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 1/24 in. in the very smallest, to ⅛ in. in large bells. Sometimes (but only in very badly made instruments) the armature adheres to the poles of the electro-magnet. This is due to residual