CURRENT FOR ELECTRIC LAMPS.
This may be either direct or alternating. The former is commonly used for arc lamps, the latter for incandescent, as it is easily stepped-down from the high-pressure mains for use in a house. Glow-lamps usually take current of 110 or 250 volts pressure.
In arc lamps fed with direct current the tip of the positive carbon has a bowl-shaped depression worn in it, while the negative tip is pointed. Most of the illumination comes from the inner surface of the bowl, and the positive carbon is therefore placed uppermost to throw the light downwards. An alternating current, of course, affects both carbons in the same manner, and there is no bowl.
The carbons need frequent renewal. A powerful lamp uses about 70 feet of rod in 1,000 hours if the arc is exposed to the air. Some lamps have partly enclosed arcs—that is, are surrounded by globes perforated by a single small hole, which renders combustion very slow, though preventing a vacuum.
ELECTROPLATING.
Electroplating is the art of coating metals with metals by means of electricity. Silver, copper, and nickel are the metals most generally deposited. The article to be coated is suspended in a chemical solution of the metal to be deposited. Fig. 84 shows a very simple plating outfit. A is a battery; B a vessel containing, say, an acidulated solution of sulphate of copper. A spoon, S, hanging in this from a glass rod, R, is connected with the zinc or negative element, Z, of the battery, and a plate of copper, P, with the positive element, C. Current flows in the direction shown by the arrows, from Z to C, C to P, P to S, S to Z. The copper deposited from the solution on the spoon is replaced by gradual dissolution of the plate, so that the latter serves a double purpose.
Fig. 84.—An electroplating outfit.
In silver plating, P is of silver, and the solution one of cyanide of potassium and silver salts. Where nickel or silver has to be deposited on iron, the article is often given a preliminary coating of copper, as iron does not make a good junction with either of the first two metals, but has an affinity for copper.
[17] From the Latin commuto, "I exchange."