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THE LIGHT SOURCE 377
flame arc carbon. The cores contain the cerium group compounds which, as we already know, supply a very brilliant, white light.
(107) Instead of permitting the arc voltage to increase with increasing current ilow, as in the normal operation of the old type flame arc, the voltage, as well as the arc length, is decreased in this new light source, tending to steady the arc and to concentrate the light producing areas into smaller space at the carbon tips, a is shown in Fig. 91.
Figure 91
This concentration gives a very brilliant light source, from which the light is collected by a mirror and sent to the projector aperture in the form of a "spot."
Other facts established during the development of this light source are, (108) first, that a much larger quantity of the light can be projected to a screen with a given optical system and aperture plate than with a low intensity, direct current arc taking the same power from the line. Second (109) the color of the screen light is blue-white, resembling very closely that obtained from the direct current high intensity arc.
(110) In addition to this the first cost is less because expensive switchboards and current rectifying devices are unnecessary, these being replaced by a relatively lowpriced, highly efficient transformer. The switchboard can be very simple and inexpensive. (Ill) In addition to all this no ballast resistance is required, eliminating the loss of light so common with the use of ballast re