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.

[Illustration: DIRECT CURRENT ARC

Most of the light being emitted by the positive (upper) electrode]

[Illustration: FLAME ARC

Most of the light being emitted by the flame]

[Illustration: ON THE TESTING-RACKS OF THE M