[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.
Fig. 18.
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.
Fig. 19.