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THE LIGHT SOURCE 395
(155) Extreme drafts of air, particularly those caused by projector rear shutters, passing through the lamp house disturb this type of arc. Where rear shutter blades have vanes to force the draft, the front vent holes of the lamphouse must be either well protected or closed. Some lamp manufacturers recommend the removal of the vanes, but that is merely robbing Peter to pay Paul since it increases the heat at the projector aperture.
(156) It is quite possible to protect the front lamphouse vent holes from such air currents as the rear shutter sets up. Make a metal trough of sheet metal, preferably russia iron to cover the lower holes, the trough having opening at the sides, with a protecting flap bent backwards at the ends, and another at the top, opening upward and back. It is a very simple device and requires « >nly a tinner's shears and an hour of time.
i 157) The Suprex carbons used in this light source are made in three diameters, each having a definite current range. Do not attempt to either over-load or under-load them. The 6 mm positive and 5 mm negative combination operates at from 34 to 40 amperes within an arc voltage of 29 to 31 ; the 7 mm and 6 mm unit operates at from 42 to 50 amperes with an arc voltage of 30 to 34; the 8 mm and 6.5 mm unit operates at from 55 to 65 amperes with an arc voltage of 36 to 40.
(158) In the opinion of competent authorities this light source, when efficiently handled, will produce a very much higher screen illumination per watt of applied power than the low intensity lamp. While it is still impractical to secure more illumination at the screen center with the 8 mm trim than with the 7 mm trim, there is this important difference : the 8 mm trim will supply higher illumination at the edges of the screen image than the trim of smaller diameter, and thus often gives the effect of even illumination over the entire screen area. The total light production is far below that of the rotating carbon high intensity light source but in conjunction with a specially devised optical system it is highly efficient.
(159) In addition to maintaining a straight, even