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160
BRITISH K1NEMATOGRAPHY
Vol. 17, No. 5
available, 150, 220, 260 and 300 amps, (the last corresponding to 20 Kw. in the arc), selected by a switch on the lamp which operates the contactors remotely. The striking and feeds of both positive and negative carbons are automatic. An optical device is fitted to maintain the positive crater at the correct distance from the first condenser lens, and its position never varies by more than ±1/32 inch. Trays are fitted beneath the lamp to hold new and used carbons. All the mechanical parts of the lamp "are completely silent. The arc noise depends upon the current, but has never given trouble with the sound-recording when using this projector in an ordinary stage ; at 150 amps, nothing can be heard when standing at the projector ; at 220 amps, there is a slight deep rushing noise which is increased somewhat at 260 amps. At 300 amps, a slight secondary flame develops from the crater, which contributes some noise, but it is not loud enough to be audible at the microphone over the set. The steadiness of the light output is particularly good, being perfect on the 150, 220 and 260 amp. settings.
TO WATER MAINS
QUARTZ
heat a9sorb'ng glass
\
CRATER
WATER PASSAGES
No. I LENS
Condenser Optical System.
The relay condenser system (Fig. 2) has been designed by Mr. A. Warmisham and Mr. J. R. Scott, and made by Taylor, Taylor and Hobson, Ltd. The primary aim has been to obtain the greatest amount of light and yet maintain full illumination at the edges of the screen — in fact the edges are slightly brighter than the centre. The first lens, recently described in detail 2> s> is only 2J ins. distant from the arc crater, its front surface being 5J ins. diameter. It is a compound lens, and also serves to filter out of the light beam almost all the unwanted infra-red radiation. The window nearest the arc is made of quartz and is cooled by a circulating current of water ; it never becomes too hot to hold one's hand against it. The water filters out some of the infra-red radiation, and the remainder is removed by a sheet of Chance Bros, type ON20 heat-absorbing glass. A further water passage assists in cooling this glass, and the light then passes into the lens. The heat taken up by the water is removed through a simple heat exchanger, cooled by tap water, which, after cooling also the arc jaws, runs to waste. Because the far infra-red has been filtered out, none of the lenses are heated unduly, and all can be made of crown optical glass.
Due to the efficient heat-filtering, the luminous efficiency of the radiation2 reaching the slide is high, 190 lumens per watt (the figure for unfiltered arc light