By the use of a quartz tube which can withstand a much higher temperature than glass, the current density can be greatly increased. Thus a small quartz tube of incandescent mercury vapor will emit as much light as a long glass tube. The quartz mercury-arc produces a light which is almost white, but the actual spectrum is very different from that of white sunlight. Although some red rays are emitted by the quartz arc, its spectrum is essentially the same as that of the glass-tube arc. Quartz transmits ultra-violet radiation, which is harmful to the eyes, and inasmuch as the mercury vapor emits such rays, a glass globe should be used to enclose the quartz tube when the lamp is used for ordinary lighting purposes.
It is fortunate that such radically different kinds of light-sources are available, for in the complex activities of the present time all are in demand. The quartz mercury-arc finds many isolated uses, owing to its wealth of ultra-violet radiation. It is valuable as a source of ultra-violet for exciting phosphorescence, for examining the transmission of glasses for this radiation, for sterilizing water, for medical purposes, and for photography.
X
THE ELECTRIC INCANDESCENT FILAMENT LAMPS
Prior to 1800 electricity was chiefly a plaything for men of scientific tendencies and it was not until Volta invented the electric pile or battery that certain scientific men gave their entire attention to the study of electricity. Volta was not merely an inventor, for he was one of the greatest scientists of his period, endowed with an imagination which marked him as a genius in creative work. By contributing the electric battery, he added the greatest impetus to research in electrical science that it has ever received. As has already been shown, there began a period of enthusiastic research in the general field of heating effects of electric current. The electric arc was born in the cradle of this enthusiasm, and in the heating of metals by electricity the future incandescent lamp had its beginning.
Between the years 1841 and 1848 several inventors attempted to make light-sources by heating metals. These crude lamps were operated by means of Grove and Bunsen electric cells, but no practicable incandescent filament lamps were brought out until the development of the electric dynamo supplied an adequate source of electric current. As electrical science progressed through the continued efforts of scientific men, it finally became evident that an adequate supply of electric current could be obtained by mechanical means; that is, by rotating conductors in such a manner that current would be generated within them as they cut through a magnetic field. Even the pioneer inventors of electric lamps made great contributions to electrical practice by developing the dynamo. Brush developed a satisfactory dynamo coincidental with his invention of the arc-lamp, and in a similar manner, Edison made a great contribution to electrical practice in devising means of generating and distributing electricity for the purpose of serving his filament lamp.
DIRECT CURRENT ARC
Most of the light being emitted by the positive (upper) electrode