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A kilowatt hour is the quantity of energy furnished in one hour by a current whose rate of expenditure of energy is a kilowatt.

342. Incandescent lamps. The ordinary incandescent lamp (Fig. 293) consists of a tungsten filament heated to incandescence by an electric current.

Since the filament would burn up in a few seconds in air, it is placed in a highly exhausted bulb. When in use it slowly vaporizes, depositing a dark, mirrorlike coating of metal upon the inner surface of the bulb. The lead-in wires are soldered one to the base A of the socket and the other to its rim B, these being the electrodes through which the current enters and leaves the lamp. The wires w, w, sealed into the walls of the bulb, must have the same coefficient of expansion as the glass to prevent leakage of air.



FIG. 293. The tungsten vacuum lamp

Incandescent lamps are usually grouped in parallel or multiple, on a circuit that maintains a potential of something over 100 volts between the terminals of the lamps (Fig. 318). The rate of consumption of energy is about 1.25 watts per candle power for the ordinary sizes. Tungsten filaments, being operated at a much higher temperature than is possible with the now almost obsolete carbon filament, have an efficiency nearly three times as great.

A customer' usually pays for his light by the kilowatt. hour (§ 341). The rate at which energy is consumed by a lamp carrying ampere at 100 volts is 25 watts. Two such lamps running for 4 hours would, therefore, consume 2 × 4 × 25 = 200 watt hours = .200 kilowatt hour. The energy is measured and recorded on a recording watt-hour meter (Fig. 321).

By filling the bulb with nitrogen a very efficient form of the tungsten lamp is obtained. The long filament is wound into an exceedingly fine spiral to minimize heat radiation. As we have already learned (§ 207), the presence of gas retards evaporation; hence, because of the nitrogen the filament may be raised to a higher temperature than is permissible in a vacuum. A greatly increased candle power results from the slight increase in current. Moreover, the convection currents in the gas-filled lamp cause the mirror due to vaporization to form near the top of the globe, where it does not obscure the intensity of the light. The larger sizes of gas-filled lamps consume only .6 watt per candle power.


343. The arc light. When two carbon rods are placed end to end in the circuit of a powerful electric generator, the carbon about the point of contact is heated red-hot. If, then, the ends of the carbon rods are separated one-fourth inch or so, the current will still continue to flow, for a conducting layer of incandescent vapor, called an electric arc, is produced between the poles. The appearance of the arc is shown in Fig. 294. At the + pole a hollow, or crater, is formed in the carbon, while the carbon becomes cone-shaped, as in the figure. The carbons are consumed at the rate of about an inch an hour, the + carbon wasting away about twice as fast as the one. The light comes chiefly from the + crater, where the temperature is about 3800° C., the highest attainable by man. All known substances are volatilized in the electric arc.

The open arc requires a current of 10 amperes and a P.D. between its terminals of about 50 volts. Such a lamp produces about 500* candle power, and therefore consumes energy at the rate of about 1 watt per candle power. The light of the arc lamp is due to the intense heat developed on account of resistance, not to actual combustion, or burning. Nevertheless, in the open arc the oxygen of the air unites so rapidly with

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FIG. 294. The arc light

*This is the so-called "mean spherical" candle power. The candle power in the direction of maximum illumination is from 1000 to 1200.

the carbon at the hot tips that in a few hours the rods are consumed. To overcome this difficulty the inclosed arc (Fig. 295) is used. Shortly after the arc is "struck" the oxygen in the inner globe is used up and then the hot carbon tips are surrounded by an atmosphere of carbon dioxide and nitrogen. Under these conditions the carbons last 130 to 150 hours. The inclosed arc is much longer than the open arc, and therefore in this lamp the P.D. between the tips is greater, usually about 80 volts, while the rest of the P.D. of the line is taken up in the resistance coils of the lamp.

The recently invented flaming arc, produced between carbons which have a composite core consisting chiefly of carbon and fluoride of calcium, sometimes reaches an efficiency as high as .27 watt per candle power. It gives an excellent yellow light, which penetrates fog well.

344. The arc light automatic feed. Since the two carbons of the arc gradually waste away, they would soon become so far separated that the arc could no longer be maintained were it not for an automatic feeding device which keeps the distance between the carbon tips very nearly constant. Fig. 296 shows the essential features of one form of this device. When no current flows through the lamp, gravity holds the carbon tips at e together; but as soon as the current is thrown on, it energizes the magnet coils m, m, which draw up the U-shaped iron core, thus striking the arc at e. As the carbons slowly waste away, the arc becomes longer, the resistance greater, and the current less; hence the upward magnetic pull weakens and the upper carbon descends, and vice versa. From time to time the upper carbon slips down through the friction clutch c. It is clear, therefore, that this automatic device will maintain that particular length of arc for which

FIG. 295. Mechanism of a direct-current inclosed arc lamp



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FIG. 296. Feeding device for arc lamp

equilibrium exists between the effect of gravity pulling down and magnetism pulling up. A dashpot d, containing a stationary piston, prevents the magnetic pull from suddenly drawing the tips at e too far apart.

345. The Cooper-Hewitt mercury lamp. The Cooper-Hewitt mercury lamp (Fig. 297) differs from the arc lamp in that the incandescent body is a long column of mercury vapor instead of an incandescent solid. The lamp consists of an exhausted tube three or four feet long, the positive electrode at the top consisting of a plate of iron, while the negative electrode at the bottom is a small quantity of mercury. Under a sufficient difference of potential between these terminals a long mercury-vapor arc is 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. CooperHewitt lamps having quartz tubes are used for sterilizing purposes because of the powerful ultra-violet rays which the quartz transmits.

FIG. 297. The Cooper-Hewitt mercury-vapor arc lamp


1. 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?

2. 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?

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


FIG. 298

4. If a storage cell has an E.M.F. of 2 volts and furnishes a current of 5 amperes, what is its rate of expenditure of energy in watts? 5. Fig. 298 shows the connections for a lamp L which can be turned on or off at two different points a or b. Explain how it works. 6. How many 100-volt lamps each carrying ampere may be maintained on a circuit where the total power may not exceed 600 watts? 7. What will it cost to use an electric laundry iron for 6 hours if it takes 3.5 amperes on a 104-volt circuit, the cost of current being $.09 per kilowatt hour?

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

9. How many lamps, each of resistance 20 ohms and requiring a current of .8 ampere, can be lighted by a dynamo that has an output of 4000 watts?

10. 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?

11. How many cells working as in problem 4 would be equivalent to 1 H.P.? (See § 144, p. 122.)

12. Since one calorie is equal to 42,000,000 ergs, 1 watt (10,000,000 ergs per second) develops in one second .24 calories. Therefore the number of calories, H, developed in t seconds by a current of I amperes between two points whose P.D. is V volts is expressed by the equation H = I×V× tx .24.

How many calories per minute are given out by the electric toaster of problem 8?

13. From the equation of problem 12 show that

H = 12Rx t x .24.

14. How many minutes are required to heat 600 g. of water from 15° C. to 100° C. by passing 5 amperes through a 20-ohm coil immersed in the water?

15. Why is it possible to get a much larger current from a storage cell than from a Daniell cell?

16. If an automobile is equipped with 6-volt lamps, how many lead storage cells must be on the car? Are these cells in series or multiple?

17. 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?

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