forms a momentary continuous conducting column of mercury, which on breaking establishes an arc the heat of which fills the tube with the vapor of mercury. A long mercury-vapor arc is thus formed, which stretches from terminal to terminal in the tube. This arc emits a very brilliant light, but it is almost entirely wanting in red rays. The strength of its actinic rays makes it especially valuable in photography. Its commercial efficiency is about .6 watt per candle power. Cooper-Hewitt lamps having quartz tubes are used for sterilizing purposes because of the powerful ultra-violet rays which the quartz transmits.

SUMMARY. Electrical power is measured in watts (§ 145). 746 watts = 1 H. P. (§ 144). Watts = volts x amperes.

Electrical energy is measured in kilowatt hours.

The number of calories of heat developed by the electric current in any number of seconds = .24 I2 Rt.

QUESTIONS AND PROBLEMS

1. If an automobile storage battery has an E.M.F. of 6 volts and furnishes a momentary current of 200 amperes in starting the engine, what is its power, or rate of expenditure of energy, in watts? in kilowatts? in horse power?

2. If one of the wire loops in a tungsten lamp is short-circuited, what effect will this have on the amount of current flowing through the lamp? on the brightness of the filament? on the life of the lamp?

3. What is meant by a 104-volt lamp? What would happen to such a lamp if the P.D. at its terminals amounted to 500 volts? Trolley cars are usually furnished with current at about 500 volts; how would you use 100-volt lamps on such a circuit?

4. A 50-volt carbon lamp carrying 1 ampere has about the same candle power as a 100-volt carbon lamp carrying ampere. Explain why.

5. A very common electric lamp used in our homes is marked 25 watts and carries about ampere. One fresh dry cell on short circuit will deliver 30 or more amperes. Will the cell light the lamp? (Prove your answer is right by using the data given.)

* Supplementary questions and problems for Chapter XIV are given in the Appendix.

6. How many 25-watt incandescent lamps, connected in parallel, might be used at the same time on a 110-volt houselighting circuit capable of furnishing a current of 10 amperes?

7. An electric flatiron taking 5 amperes from a 110-volt circuit is used for 25 min. (1) What is its resistance? (2) How many watts does it require? (3) If the rate is $.12 per kilowatt hour, what is the cost of the electrical energy used?

8. How long can you burn one 25-watt lamp in your home for a cent at $.10 per kilowatt hour?

9. A small arc lamp requires a current of 5 amperes and a difference of potential between its terminals of 45 volts. What resistance must be connected in series with it in order to use it on a 110-volt circuit?

10. The resistance of a certain water-heating coil is 20 ohms, and when connected to the house circuit it carried 5 amperes. How many minutes will it take this coil to heat 480 g. of water from 10° C. to 100° C.?

11. A certain kind of electric toaster takes 5 amperes at 110 volts. It will make two pieces of toast at once in 3 min. (1) At what horse-power rate does the toaster convert electrical energy into heat energy? (2) At $.08 per kilowatt hour what does it cost to make 12 pieces of toast?

CHAPTER XV

INDUCED CURRENTS

THE PRINCIPLE OF THE DYNAMO AND MOTOR

347. Current induced by a magnet. Let 100 or less turns of No. 22 copper wire be wound into a coil C (Fig. 303) about two and a half inches in diameter. Let this coil be connected into circuit with a sensitive d'Arsonval galvanometer, or even a simple detector made

by suspending in a box, with No. 40 copper wire, a coil of 200 turns of No. 30 copper wire (see Fig. 311). Let the coil C be thrust suddenly over the N pole of a strong horseshoe magnet. The deflection of the pointer p of the galvanometer will indicate a momentary current flowing through

S
N

a

FIG. 303. Induction of electric currents by magnets

the coil. Let the coil be held stationary over the magnet. The pointer will be found to come to rest in its natural position. Now let the coil be removed suddenly from the pole. The pointer will move in a direction opposite to that of its first deflection, showing that a reverse current is now being generated in the coil.

We learn, therefore, that a current of electricity may be induced in a conductor by causing the latter to move through a magnetic field, while a magnet has no such influence upon a conductor which is at rest with respect to the field. This discovery, one of the most important in the history of science,

was announced by the great Faraday in 1831. From it have sprung directly most of the modern industrial developments of electricity.

348. Direction of induced current. Lenz's law. In order to find the direction of the induced current, let a very small P.D. from a galvanic cell be applied to the terminals A and B (Fig. 303), and note the direction in which the pointer moves when the current enters, say, at A. This will at once show in what direction the current was flowing in the coil C when it was being thrust over the N pole. By a simple application to C of the right-hand rule (§ 309) we can then tell which was the N and which the S face of the coil when the induced current was flowing through it. In this way it will be found that if the coil was being thrust over the N pole of the magnet, the current induced in the coil was in such a direction as to

N

N

(1)

(2)

(3)

N.

S

(4)

FIG. 304. Illustrating Lenz's law

make its lower face an N pole during the downward motion (Fig. 304, (1) and (2)), and an S pole during the upward motion (Fig. 304, (3) and (4)). In the first case the repulsion of the N pole of the magnet and the N pole of the coil tended to oppose the motion of the coil while it was moving from a to b, and the attraction of the N pole of the magnet and the S pole of the coil tended to oppose the motion while it was moving from b to c. In the second case the repulsion of the two N poles tended to oppose the motion from c to b, and the attraction between the N pole of the magnet and the S pole of the coil tended to oppose the upward motion from b to a. In every case, therefore, the motion is made against an opposing force.

From these experiments, and others like them, we arrive at the following law: Whenever a current is induced by the relative motion of a magnetic field and a conductor, the direction of the induced current is always such as to set up a magnetic field which opposes the motion. This is Lenz's law. This law might have been predicted at once from the principle of the conservation of energy; for this principle tells us that since an

Famous English physicist and chemist and one of the most gifted of experimenters; son of a poor blacksmith; apprenticed at the age of thirteen to a London bookbinder, with whom he worked nine years; applied for a position in Sir Humphry Davy's laboratory at the Royal Institution in 1813; became director of this laboratory in 1825; discovered electromagnetic induction in 1831; made the first dynamo; discovered in 1833 the laws of electrolysis, now known as Faraday's laws. The farad, the practical unit of electrical capacity, is named in his honor