Vincent Stephen

Production of electric light. I have explained how power can be converted into electric currents, either continuous or alternating, and I must now show how these currents can be applied to the production of light.

Fig. 15.

Arc Light

Arc lights. The current may be used to produce an arc light in the following manner:—Two carbon rods, A and B, are held by suitable means in the position shown in Fig. 15, and the two wires from a dynamo are joined respectively to A and B, the upper one always being the positive lead when a continuous current is used. When the current is sent through the circuit, it passes through the carbons A and B, which are conductors. Immediately this occurs, suitable mechanism in the lamp, being acted on by the current, or by hand in the case of search-lights, or by clock-work, moves the two carbons a small distance apart, with the consequence that a dazzling arc of light is formed between them. Mechanism to regulate carbons. If the carbons get too far apart, the mechanism brings them nearer together again, and on the delicacy with which it acts, depends the steadiness of the light. It would be useless to explain how this mechanism acts, as it is in a different form in each maker’s lamp. Some lamps suitable for alternating current. Some lamps have been constructed for use with an alternating current, but with the majority a continuous current is used. While an arc light is burning the carbons waste away, the upper one more rapidly than the lower, and the mechanism has to approach them constantly to make up for this waste.

When carbons are consumed light goes out. When the carbons are consumed as far as convenient, an automatic arrangement cuts off the current, and the light goes out; or it diverts the current to another set of carbons, which at once light up. The carbons are made in suitable lengths to last a certain number of hours, four, six, eight, &c. In Fig. 16 is shown an arc lamp complete.

Arc lamp very complicated. An arc lamp is of necessity a complicated affair, which it is not advisable to have on board ship, except where an electrician is engaged permanently.

Jablochkoff candle. Another way of producing light is to use the current in what is called an electric candle, of which a familiar type is the Jablochkoff candle.

Fig. 17 shows the form of this candle, A and B being two carbon rods parallel to one another, and joined, but at the same time insulated from one another by kaolin, a sort of chalky substance, which is a non-conductor.

The wires C and D from the dynamo are joined respectively to A and B through metallic supports, as in an arc lamp, and when the current is turned on it flows through C A and across by a small strip of carbon E to B and D back to the dynamo. Arc formed between the carbons. The strip E is only large enough to carry the current across for a moment, and is immediately consumed, but an arc of light is then formed between the carbons as in the arc lamp. As the carbons consume, the kaolin in between burns away, just in the same manner as, in an ordinary candle, the wick is consumed and the wax melts and burns away, except that in the latter case the wax feeds the light, whereas the kaolin is only used to keep the carbons the required distance apart and the arc of light from running down them.Candles require alternating current. It is evident that the carbons must be consumed equally, for which reason use must be made of the alternating current. Any unsteadiness that occurs in the light produced is consequent on unsteadiness of the current, or impurities in the carbons, &c., there being no mechanism of any kind required. These candles do not give such a great light as arc lights, but it is of the same nature in every way. Fig. 18 shows one of these candles in its holder, from which can be seen how electrical contact is made with the two carbons.

If the current is interrupted in any way, and the light goes out, it will not be produced again automatically, but requires a small piece of carbon between the two carbons as a path for the current to pass across as in the beginning.

Incandescent lamps. A third form of electric light is produced by using the current in an incandescent lamp.

To explain the action of an incandescent lamp, I must refer back to what I said about wires getting heated by a current being passed through them which was too large for their capacity. If two large wires are joined by a small one, and a strong current is passed through the circuit, the small wire rapidly gets red hot, and finally fuses. Vacuum formed in lamp prevents combustion If this small wire is contained in a globe from which the air is exhausted, when the current is passed through it, it gets red, then white hot, and when very brilliant gets fused. If, instead of wire, we have in the small globe a thin filament of carbon, when the current is passed through, we get a brilliant light which remains constant because the carbon does not fuse, and it cannot burn away for want of air. Fig. 19 shows a Swan lamp, and Fig. 20 an Edison lamp, both made on this principle.

Vacuum not perfect. If in these lamps the vacuum were perfect, the carbon filament would never get consumed; it is, however, impossible to get a perfect vacuum, but the better it is, the longer will the filament last. Incandescent lamps are the only ones that are suitable for house or ship lighting. Advantages of incandescent lamps for house and ship lighting. They give a yellowish light like a good gas-flame, they do not consume the air of a room, they cause no smell, and only give out a very slight heat. They are perfectly safe, because if the globe gets broken and allows air to get in, the filament is instantly consumed, and the light goes out. They can be put in all sorts of places where it would be impossible to have any other lamps, such as near the ceiling, close to curtains, in a room full of explosives or combustibles, and even under water. Unaffected by wind, and suitable for either continuous or alternating currents. They are not affected by wind; they can therefore be used under punkahs, or near open windows, sky-lights, or ports, or in the open air. These lamps can be used with either continuous or alternating currents, but will probably last longer with the latter, because, when a continuous current is used, particles of the carbon of the filament appear to be conveyed from one end of the filament to the other, reducing the thickness at the one end, until finally it breaks. This evidently cannot occur with an alternating current, as the impulse in one direction is counteracted by the following one in the opposite direction. If the current used is of too high a tension for the lamps, they will show an intensely brilliant light for a short time, but the filament will soon be destroyed, and the lamp rendered useless.