International projectionist (Jan 1959-Dec 1960)

lifters, however, do not introduce noticeable flicker into the projection light where AC input voltage is constant. It is only in certain rural areas having faulty mains current that the use of rectifiers causes severe picture flicker. Carbon-Feeding Devices Once the correct arc current, voltage, and arc-gap distance have been established, the lamp carbon-feeding mechanism should preserve these factors unchanged. The feed mechanisms for most of the smaller and older lamps are current-operated devices that maintain a constant arc current and, in consequence, a fairly constant arc gap. The feed mechanism's job has been made more difficult by fastburning H-I carbons. The old-style intermittent-feed type of control utilized a relay switch connected in parallel with the arc. As the arc gap increased, due to the burningaway of the carbons, more current passed through the relay electromagnets which turned the feed motor off Panoram Par-Excellence In the Philips pavilion at the Brussels World Fair performances were given of "Electronic Poem," a combination of light effects and electronic music involving extensive technical provisions. The equipment for the light effects included four large cinema projectors, eight projection lanterns, six spotlights, six ultra-violet lamps, 50 electric bulbs representing stars, and several hundreds of tubular fluorescent lamps in various colors. The luminous flux of most of these lightsources is variable by means of 30 thyratron control circuits. The electro-acoustical installation comprised a special playback machine with a three-track perforated magnetic tape on which is recorded a composition by Edgar Varese, 20 amplifiers each with an output of 120 W. 350 loudspeakers, and an elaborate switching system with relayed and telephone selectors. The electronic control system employed a second playback machine. which scanned a 15-track perforated magnetic tape. Twelve control signaldiffering in frequency are recorded on each track. >o that 180 channels are available for controlling the light-and-sound effects in accordance with the scenario. Both playhack machines are duplicated, with a view to providing a reserve as well as ensuring continuous performances. Consistent Maximum Light at Greater Economy DIAMOND PROJECTION 70 PINE STREET • NEW YORK 5, N.Y and on. With sufficient current, the relay switch closes, causing the feed to operate. When the arc gap is thus reduced in length, the relay opens and the feed motor stops. Dirty relay contacts and incorrect spring tension were the chief causes of feeding trouble. Most Suprex-type lamps employ a special feed motor having a heavycurrent field winding connected in series with the arc. Lengthening of the arc gap as the carbons burn away causes a decrease of current through the control field of the motor. The motor then speeds up. As the gap shortens, the current through the control field becomes stronger, and the speed of the motor decreases. Modern high-powered lamps employ optical crater-positioning feeding mechanisms. An image of the crater is focused either upon a bimetallicstrip thermostat or upon a photoelectric cell. Operation of the thermostat or photocell by the action of either heat (infrared rays) or light emanating from the crater controls the carbon-feeding motor and maintains correct crater position within remarkably close limits. "Photronic" arc control is undeniably the most satisfactory for fast-burning, high-powered H-I carbon trims. While fully automatic carbon feeding by photronic controls is independent of arc-current variations, the older relay and control-field devices are extremely sensitive to all electrical factors involved in the burning of an arc. Each is fitted with an adjustment which permits the projectionist to vary, within reasonable limits, the current which actuates the feeding mechanism. In the case of the relay type of control, the adjustment is made in the tension of the relay contact spring against which the electromagnet works; in the case of the continuousoperation feed motor of simplified H-I lamps, a manually operated shunt-field rheostat is used. It should be kept in mind that any defect in the power sup ply or arc circuit which causes an acci dental reduction in current will result in fast carbon feeding and a radically shortened gap with either of these two feed devices. Separate Negative Feed It should also be remembered that a single ratio of negative feed-rate to positive feed-rate works satisfactorily at one amperage only. If the current be increased, the negative carbon will burn faster in relation to the positive burning rate, and the feed mechanism will cause the entire arc to creep slowly toward the mirror. Likewise, a decrease of current causes the arc to creep slowly away from the mirror even though the correct arc gap is maintained at all times. This is one of the difficulties of operating older lamps, such as the Simplex High, which had no separate negative-feed adjustment. The only way to make these lamps burn properly without an undue amount of manual adjustment is to regulate the arc current very carefully by means of the generator field rheostat. Rapid arc creeping caused by abnormally high or low current calls for a little experimenting with larger or smaller negatives. Correct setting of the arc-feed mechanism, including the separate negative speed control, is undoubtedly the most critical electro-mechanical aspect of arc-lamp operation. But once these adjustments have been properly made, there should be little need to alter them radically unless the arc current is changed or different sizes of carbons are used. The necessity for clean, properly lubricated lamp mechanisms and good electrical connections throughout is well known to all projectionists. 22 INTERNATIONAL PROJECTIONIST JANUARY 1959