343. Amount of heat developed by a current. Since one calorie is equal to 42,000,000 ergs (§ 185), 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 = .24 IVt.

Since from Ohm's law V = IR,

H = .24 12Rt,

or the heat generated in a conductor is proportional to the time, to the resistance, and to the square of the current.

344. Incandescent lamps. The ordinary incandescent lamp (Fig. 299) consists of a tungsten filament heated to incandescence by an electric current.

wy Sw

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 leadin 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. 299. 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. 321). 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.

THE EARLY EDISON LAMP AND THE MODERN GAS-FILLED LAMP In 1879 Thomas A. Edison invented a carbon-filament vacuum lamp of high resistance. To operate these lamps in parallel (the only practical method for homes) he invented a three-wire, constant-potential, generating and distributing system, by which the current was subdivided. This system is the one in use today. In 1911 Dr. William D. Coolidge, after years of work, discovered how to produce ductile tungsten for the manufacture of filaments, and in 1913 Dr. Irving Langmuir invented the extremely efficient, gas-filled, tungsten lamp. These two notable triumphs made the incandescent lamp supreme as the presentday illuminant, saving the public over a billion dollars on what it would have to pay if the amount of light used at this time was obtained from the obsolete carbon lamps. The picture on the left shows one of Edison's very early, commercial, carbon-filament vacuum lamps. It was placed on the market in 1880, was 7 inches long, consumed 100 watts and gave about 16 horizontal candle power, or 160 lumens. The lamp pictured on the right represents the latest development of the gas-filled, spiral-filament type. The one here shown is 6 inches long and also consumes 100 watts but gives about 132 candle power, or 1320 lumens. The gas used is argon mixed with a small percentage of nitrogen, since pure argon ionizes at the temperature of operation and hence creates a short circuit. The globe is smooth on the outside and frosted within, the inner irregularities being filled with a transparent medium which enables the glass to transmit practically all the light. From 6 per cent to 20 per cent of the light is absorbed by lamps frosted on the outside. There are upward of half-abillion incandescent lamps in use in the United States, which is about half the number used by the entire world. (Courtesy of the Edison Lamp Works)

Resistance devices are used to transform electrical energy into heat and to control the strength of currents. For example, electric stoves, toasters, soldering irons, water heaters, laundry irons, and fuses utilize the heating effect of the current, whereas motor starting boxes and theater dimmers are used in current control. The heating element of the iron shown in the figure consists of nickel-chromium ribbon wound upon a sheet of mica. An ordinary iron requires about 600 watts, or enough power to operate ten 60-watt lamps. The figure at the right shows the principle of resistance dimmers used in theaters

Great care must be taken to remove not only the air but traces of water vapor which adheres with great persistence to the surface of the glass. To remove this the globe is heated to at least 300° C., red phosphorus in small amount being present. A vacuum of .001 mm. is thus obtained. The life of these lamps is usually from 1000 to 2000 hours.

Black glass

Brass contacts

A customer usually pays for his light by the kilowatt hour (§ 342). 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 kilo

=

Tube through

which air was

exhausted and

watt hour. The energy is argon admitted
measured and recorded on
a recording watt-hour meter
(Fig. 324).

-Cement

Magnified spiral filament in support

-Spiral

tungsten

filament

FIG. 300. Gas-filled lamp with fine spiral filament

By filling the bulb with argon a very efficient form of the tungsten lamp is obtained (Fig. 300). The long filament is wound into an exceedingly fine spiral to minimize heat radiation. As we have already learned (§ 206), the presence of gas retards evaporation; hence because of the argon the filament may be raised to 2400° C., 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 gasfilled 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. (See opposite page 308.)

Today the production of incandescent lamps is enormous. Under the patents of one organization alone there are manufactured 3000 lamps every minute of every working day throughout the year.

345. The carbon 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 onefourth 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. 301. At the + pole a hollow, or crater, is formed in the carbon, and 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 3700° C. (6800° F.), the highest attainable by man. All known substances are volatilized in the electric arc.

The light of the arc lamp is due to the intense heat developed on account of resistance, not to actual combustion, or burning.

The powerful and efficient arc lamp is now practically obsolete except for spot lights, moving-picture machines, and search lights. 346. The Cooper-Hewitt mercury arc lamp. The CooperHewitt mercury arc lamp (Fig. 302) differs from the carbon 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 being a plate of

FIG. 302. The Cooper-Hewitt mercury-vapor

arc lamp

iron, and the negative electrode at the bottom a small quantity of mercury. Before the lamp begins to burn, the space within it is an almost perfect vacuum, through which the difference in potential at the terminals is unable to send a current. To start the lamp the lower end is tilted upward and then immediately lowered. This