International projectionist (Jan 1963-June 1965)

considered to be in good condition only when this reddish-brown oxide film is present! Generators Good, But Wasteful Motor-generator sets are still retained in many theatres using simplified HI arc lamps. Motor-generators have the advantage of delivering extremely smooth direct current relatively unaffected by AC line-voltage fluctuations. This is important in neighborhoods where the main current is poorly regulated. But motor-generators are noisy, they are relatively costly to purchase and to maintain in good working order, and they require the use of current-wasting ballast rheostats. Even though a motor-generator set, considered by itself, may have the same 85% power-converting efficiency of a selenium, tube-type, or silicon diode rectifier, it is much less efficient than a rectifier in actual service because of the unavoidable ballast losses. The voltage drop across a ballast rheostat (when the arc is burning normally) is the difference between the generator output voltage (indicated by the voltmeter in the control cabinet) and the actual voltage drop across the arc. This difference should never be less than 15 volts for the smallest simplified HI ("Suprex") arcs, or less than 25 volts for the most powerful rotatingpositive HI arcs. An excessively large ballast drop insures stable burning of the arcs, but also wastes electric power unnecessarily. In general, the ballasts should be adjusted so that the arcs burn at their maximum rated current or, better, at 2 or 3 amperes above the rated maximum. Then the generator voltage is slightly decreased by means of the field rheostat (usually located in the generator control cabinet) until the current drawn by the normally burning arc is at the desired value in the current range recommended by the carbon manufacturer for the size of trim being used. The voltage drop occasioned by the very slight resistance of the transmission line is so small that it may be ignored. "Creeping" Arcs in Suprex Lamps Certain lower-priced simplified HI arc lamps have a fixed positive-negative carbon feed ratio. The lamp manufacturer has assumed that the positive carbon will be consumed just so much faster than the negative carbon, and accordingly has threaded the carbon-feed shafts Positive electrode UXl ijiary^ mirror ?1 Film aperture Negative electrode PIG. 3 —The optical system of a xenon-bulb light source is similar to the optics of a carbon-arc reflector lamp. The spherical auxiliary mirror, not used in a carbon arc lamp except the "blown-arc" type, nearly doubles the luminous output of the lamp and smoothes the screen illumination by superimposing an inverted image of the xenon arc upon the light source, itself. Unlike the carbon arc, which emits most of its light from a crater in the tip of the positive electrode, the xenon lamp emits its light from the gaseous discharge between two tungsten electrodes. r + Rectifier iii 3-phase A.C. \ ,\ Xenon bulb 1 — r Igniter ' I Combination con-> © trol pushbutton for igniter and rectifier. FIG. 4 — The carbon arc has movable electrodes which are brought together and then separated to establish a currentconducting arc, but the xenon bulb has fixed electrodes. The low-voltage, high-amperage current needed for this lamp cannot pass until a conducting path has been established through the xenon gas by a high-voltage spark. This is furnished by an ignition device which automatically cuts in the low-voltage operating circuit the moment a spark jumps between the electrodes. Actual operation of a xenon projection lamp is extremely simple — just press a button! No optical adjustments are necessary once the long-life bulb has been installed and "lined up". to correspond with this particular ratio. Actually, this ratio (whatever it may be) obtains only at one definite arc current. Other positive-negative feed ratios are observed at other arc currents even when the same trim of carbons is used. If the current be decreased or increased in a lamp having a fixed feed ratio, the positive-negative burning ratio will be changed. No way is provided for the projectionist to compensate for the change in such a lamp: all he can do is make sure that the current is maintained at a value which will hold the arc in focus. Suppose that the current is increased for some reason, perhaps to get a brighter picture or to get a whiter, more efficient light. The rate of positive consumption then increases over the rate of negative consumption relative to the feed ratio at which the lamp was set at the factory. What happens? The entire arc creeps out of focus away from the mirror, and the light on the screen gradually becomes dim and bluish. The reverse happens when the current is decreased, perhaps for the purpose of saving money on carbons — a false economy, by the way. The positive burning rate decreases over that of the negative, and the entire arc creeps toward the mirror. The screen light then becomes dim and brownish. If a radical increase or decrease in arc current is desired in a fixed feed-ratio Suprex lamp, it is best to use the next larger or smaller trim of carbons. This is sometimes possible without having to replace the carbon holders, but not always. At all events, we suggest that the size of trim chosen be burned as close to the maximum rated current as possible in order to get the brightest, whitest light. It has been determined by actual test that a 7 and 6-mm Suprex (copper-coated) trim burned at 50 amps, gives fully 30% more screen light than an 8 and 7-mm Suprex trim burned at 60 amps! There is more leeway in the choice of arc currents for burning any particular Suprex trim if the lamp has a separate feed control for the negative carbon. Even so, it is best to hang pretty close to the maximum rated current for the carbons in order to obtain the kind of light that pleases the cash customers. Arc Gap in Rotating HI Lamps Higher-powered HI arc lamps — those having rotating positive carbons — respond to current variations in a (Continued on Page 11) International Projf.ctionist March 1963