[Fig. 168] shows the two coils, P, S, of the induction coil. The primary, P, is joined in series with a cell[207] and transmitter. The secondary coil, S, is joined to the receiver. One end of S can be grounded, the current completing the circuit through the earth and into the receiver through another wire entering the earth. There are many forms of transmitters. (See "Things a Boy Should Know About Electricity.")
484. Electric Lighting and Heating. Whenever resistance is offered to the electric current, heat is produced. By proper appliances, the heat of resistance can be applied just where it is needed, and many commercial processes depend upon electricity for their success. Dynamos are used to generate currents for lighting and heating purposes. There are two great systems of lighting, the one by arc lamps and the other by incandescent lamps. (See "Things a Boy Should Know About Electricity.")
485. Arc Lamps produce a light when a current passes from one carbon rod to the other across an air-space. As the current starts through the lamp, the ends of the carbons touch, and the imperfect contact causes resistance enough to heat the ends red-hot. They are then automatically separated, and the current passes from one to the other, causing the "arc." The resistance of the air-space is reduced by the intensely heated vapor and flying particles of carbon.
486. The Incandescent Lamp consists of a glass bulb, in which is a vacuum, and the light is caused by the passage of a current through a thin fibre of vegetable carbon, enclosed in the vacuum. The fibre would burn instantly if allowed to come in contact with the air. The fibres have a high resistance, and are easily heated to incandescence.
CHAPTER XXVIII.
WIRE TABLES.
Copper Wire Tables are very convenient, and a necessity when working electrical examples. The tables here given are taken from a dealer's catalogue, and will be found sufficiently accurate for ordinary work.