524 MAGNETISM AND ELECTRICITY with slight modifications, was used. A few such lamps are even now in use. In 1906 the carbon filament was replaced by the more economical tantalum and tungsten filaments, which can be heated to higher temperatures and yield a whiter light. The early tungsten-filament lamps were not very successful because the tungsten filament was so brittle and fragile that it broke easily and because it vaporized so rapidly that the lamps had very short lives. W. D. Coolidge, of the Research Laboratory of the General Electric Company, worked for many years trying to accomplish the "impossible" feat of making tungsten ductile. At last his indomitable perseverance, backed by excellent facilities, brought the triumph that made the modern, inexpensive, economical electric lamp possible. This invention did not solve all of the lamp manufacturer's problems. The bulbs began to blacken soon and grew blacker with use. It was found that the blackening produced by the vaporization of the tungsten filament and its subsequent deposition on the glass could be greatly decreased by adding an inert gas like nitrogen or argon. This caused a decrease in the efficiency of the lamps due to the heat transmitted by the gas, but this was more than overcome, in turn, by using concentrated filaments or coils of filament. On the other hand, higher temperatures were made possible, and a better, whiter light was produced. The modern gasfilled tungsten-filament lamp uses about one-fifth to one-sixth as much power as the carbon-filament lamps for the production of the same amount of light. Progress is still being made in the production of lamps with longer lives which produce more light at lower cost. For example, a simple change in the construction of 60-watt lamps in the United States alone gave users $12,000,000 worth of additional light for their money in a single year. Incandescent lamps of considerable power have been developed so that 500- to 1000-watt lamps have now largely replaced arc lights in projection machines. Lamps of even greater power are used for certain purposes. Two recent types of lamps are the photoflood lamp and the photoflash lamp. The photoflood lamp gives an intense white light, with some ultraviolet radiations, and it can be used as a source of such radiations. When photoflood lamps are used in the ordinary lighting circuits, the filaments are heated to an intense white heat. In this case the intensity of the light is more important than the length of life of the lamp, which is only a few hours. The photoflash lamp contains thin aluminum foil or wire, which burns with a flash in the oxygen gas in the bulb when the current is turned on; the current from a small dry cell is all that is required.
ELECTRICITY MAY BE CONVERTED INTO HEAT 525 Operating Economy of Electric Lamps Depends upon the Voltage of the It is Circuit in Which They Are Used. important to select electric lamps which are designed for use with a given lighting circuit. If a 110-volt lamp is used with a 120-volt circuit, it will give about one-third more light than it would with a 110- volt circuit; but its life will be only about one-third as long because the temperature to which the filament is subjected is too high. On the other hand, a 120-volt lamp when used with a 110-volt circuit will last about two and one-half times as long, but it will give only 74 per cent as much light as it would if it were operated at 120 volts. Electric Current May Be Produced from Heat. Just as mechanical energy can be converted into electrical energy and electrical energy can be converted back into mechanical energy, so we shall see later that the conversion of chemical energy into electrical energy can be reversed and, to a certain extent, the conversion of electrical energy into heat energy can be reversed. This latter process is not efficient, however, and has little practical application. The direct conversion of heat energy into electrical energy is called the thermoelectric effect. All metals, some nonmetals, and many compounds emit electrons when heated. When two dissimilar metals joined together are heated, there will be a flow of electrons from one metal to the other because of the difference in their tendency to emit electrons. Thermocouples and thermopiles, which consist of a series of thermocouples, serve as devices to measure temperature because the increase in current registered by a galvanometer is closely proportional to the rise in temperature for certain combinations of metals. Thermopiles are made so sensitive that they can detect the heat from distant stars which changes the temperature only 0.000,000,1° C. It is interesting to note that there is an inverse thermoelectric effect in which the temperature of a junction between two metals changes when a current passes through the junctions. Heating Effects Are Also Produced by High-frequency Currents. The General Motors Corporation exhibited a combination refrigerator and stove which -they called the Frig-o-Therm at the World's Fair in 1940. The cooking was accomplished by means of an alternating current having a frequency of 100,000 per second. Each alternation induces a current in the iron skillet and thus causes it to get hot. induction furnace works on this principle. High-frequency currents may be used to heat the human body. The diathermy treatments with high-frequency currents, very large cur- In