Motion Picture Herald (Jan-Mar 1956)

Lamps with Rotating Clamp And Photoelectric Arc Control By M. 0. FAIGE "Doc" Faige and Associates, New York THERE ARE several excellent lamps available for the larger theatres and drive-ins. The three “Gonematic” models manufactured by the Murch Electric Company of Franklin, Me., and distributed by “Doc” Faige and Associates, New York, through dealers throughout the U. S. and abroad, are among these. In external appearance the new “Conematic” lamp looks very much like most other H. I. reflector lamps of its general class. The uncoated positive carbon rotates as it feeds, and the copper-coated negative is inclined at an angle of 40 degrees. The arc is readily struck with a twist of a large knob on the operating panel. There are distinguishing features of importance, however. One is that the positive carbon is firm in its holder and the whole clamp is rotated by means of a chain and sprocket assembly which also rotates the positive feed screw pusher mechanism. Distribution of light on the screen has long been a bone of contention in projection circles. Side-to-center distribution on the screen with the “Gonematic” ranges from 80 to 85 per cent when this lamp is focused for maximum efficiency. The light output is in the order of 20,000 lumens with standard optical sound film aperture and coated / 1.9 lens of 5-inch focal length, and burning the 9mm “black” positive carbon with 5 16-lnch coated negative in the 75-80 ampere range. The reflectors employed in “Gonematic” lamps are 16'/2 and 18 inches in diameter, working at geometric speeds of /, 1.9 and //1.7, respectively. The working distance may be varied over a range of about 2 inches without appreciably affecting the quantity or quality of light. Another outstanding special feature of “Gonematic” lamps is a photoelectric arc control. As projectionists well know, even with a bimetallic thermostat type of control, the arc gap may change in length, and the positive carbon may wander outside tolerated limits. Instead of a bimetallic switch, which is activated only by heat emanating from the edge of the positive crater, and which necessarily permits the positive carbon to advance or recede beyond the optimum position whenever a lip accidentally forms, the “Gonematic” photoelectric arc-feeding and craterpositioning mechanism operates by “looking” at the center of the luminous gas ball as reflected by the mirror that handles the light illuminating the picture. The design of the photoelectric arc control provides for a ray of light to pass from the center of the positive crater to the edge of the mirror, and thence to a small cylindrical reflector made of stainless metal and having a seml-dlffuse optical surface. The Murch optical cylinder, used in all “Gonematic” lamps, is supported at the end of a short length of tubing. The optical cylinder intercepts the edge of the beam thrown forward by the mirror and reflects the light to a photocell at the side of the lamphouse. This photocell is of the standard caesium type used for sound reproduction. Should the positive crater advance toward the mirror, the beam of light reflected by the mirror becomes slightly wider and results in stronger illumination of the photocell. This causes the feed motor to slow down until the crater burns back to the point of correct geometric focus. Gonversely, if the crater recedes from the mirror, the photocell gets less light because the beam has been slightly diminished in diameter. Immediately the photocell speeds up the motor and restores the crater to its proper focal position. The rate of positive feed in relation to the advance of the negative carbon is adjustable, as is also the overall speed of feeding. A correct rate of positive-carbon rotation has been established by design. Electrical contact with the positive carbon is made by wide, silverfaced brushes. A separately powered blower is built into the “Gonematic” to maintain a low operating temperature and to insure complete combustion and removal of gas. The burner remains cool enough to permit retrimming with the bare hands, even immediately after a shut-dowm. Interior view of Murch "Conematic" lamp showing rotating clamp and contact assembly, and optical cylinder (above positive carbon) of arc control system, which adjusts gap in response to mirror reflection. 26 MOTION PICTURE HERALD, JANUARY 7, 1956