323. The Telephone. When a magnet is at rest within a closed coil of wire, as in Section 319, current does not flow through the wire. But if a piece of iron is brought near the magnet, current is induced and flows through the wire; if the iron is withdrawn, current is again induced in the wire but flows in the opposite direction. As iron approaches and recedes from the magnet, current is induced in the wire surrounding the magnet. This is in brief the principle of the telephone. When one talks into a receiver, _L_, the voice throws into vibration a sensitive iron plate standing before an electromagnet. The back and forth motion of the iron plate induces current in the electromagnet _c_. The current thus induced makes itself evident at the opposite end of the line _M_, where by its magnetic attraction, it throws a second iron plate into vibrations. The vibrations of the second plate are similar to those produced in the first plate by the voice. The vibrations of the far plate thus reproduce the sounds uttered at the opposite end. [Illustration: FIG. 241.--Diagram of a simple telephone circuit.] 324. Cost of Electric Power. The water power of a stream depends upon the quantity of water and the force with which it flows. The electric power of a current depends upon the quantity of electricity and the force under which it flows. The unit of electric power is called the watt; it is the power furnished by a current of one ampere with a voltage of one volt. One watt represents a very small amount of electric power, and for practical purposes a unit 1000 times as large is used, namely, the kilowatt. By experiment it has been found that one kilowatt is equivalent to about 1-1/3 horse power. Electric current is charged for by the watt hour. A current of one ampere, having a voltage of one volt, will furnish in the course of one hour one watt hour of energy. Energy for electric lighting is sold at the rate of about ten cents per kilowatt hour. For other purposes it is less expensive. The meters commonly used measure the amperes, volts, and time automatically, and register the electric power supplied in watt hours. INDEX Absorption, of heat by lampblack, 143-144. of gases by charcoal, 57. of light waves, 135-138. Accommodation of the eye, 123. Acetanilid, 259. Acetylene, as illuminant, 152-153. manufacture of, 152-153. properties of, 220. Acid, boric, 253. carbolic, 152, 251, 252. hydrochloric, 55, 80, 227, 238, 241. lactic, 230. oxalic, 247, 248. salicylic, 253. sulphuric, 55, 80, 240, 241, 307. sulphurous, 242. Acids, action on litmus, 220. Adenoids, 51. Adulterants, detection of, 16. Air, characteristics of, 81-83, 86, 189. compressibility of, 91. expansion of, 10-11. humidity, 38, 39. pumps, 201-205. transmits sound, 269. weight of, 86. _See_ Atmosphere.