International projectionist (Jan 1963-June 1965)

LAMPS AND POWER from Page 6 different way. Most reflector lamps of this type have an automatic optical crater-positioning device which insures that the positive crater remains in the focus of the mirror at all times. If the current supplied to the trim is too low, the arc gap becomes gradually shorter than normal; if the current is too high, the gap lengthens. Some lamps have independent negative-feed controls: others have positive-negative feed rate selectors for a number of different trims and currents. The optimum length of arc gap (the distance between the tips of the two carbons) is about % inch for arc currents between 40 and 50 amps., % inch for currents between 60 and 120 amps., and % inch for currents between 130 and 180 amps. A variation in gap length not exceeding 1/16 of an inch does not appear to be important. However, the use of an excessively long arc gap may allow the arc to waver, causing the screen illumination to flicker in an annoying manner. To sum up: If your arcs burn in an unstable manner, with erratic feeding of the carbons, check the electrical system before blaming the lamps or the carbons for the difficulty. A cracked carbon will certainly give trouble, but damaged carbons can usually be discovered by inspection. Water does no permanent damage to carbons, but they must be thoroughly dry when burned. It is a good idea to keep a few dozen carbons under the lamphouse or in some other warm place to insure a supply of dry carbons. Check the System Methodically! How to go about checking the electrical system? A measurement of arc amperage and voltage comes first. Arc voltage drop is measured with a DC voltmeter across the lamp lead wires or table switch when the arc is burning normally. This voltage is less than generator output voltage (as indicated by the voltmeter in the control cabinet ) . and should be 15 volts less for the smaller lamps and about 25 volts less for the larger ones. This voltage difference happens to be the ballastrheostat voltage drop spoken of earlier. An ammeter is ordinarily present in the system, either in the generator control cabinet or in the lamphouse, itself. An ammeter is absolutely essential for keeping the projectionist informed as to the current being drawn by his arcs. All connections everywhere in each arc circuit are then checked for evidence of corrosion, looseness, undue heating, etc., special attention being given to the wires and terminals exposed to the heat of the arc in each lamphouse. Perfect performance of the arcs is not guaranteed by a perfect electrical system, of course, — the lamp and carbon control and feed mechanism may be worn, out of adjustment, clogged by dust, incrusted with grime, or improperly lubricated. But no matter what the age or physical condition of the lamps may be, satisfactory burning of the arcs is manifestly impossible unless the electrical factors to which the carbon arc is extremely sensitive are all in A-l order. iP There is a FIRST BEST in Anything! In ARC LAMPS IT IQ TUP ADTlfikllY IMTEAPATEn I 1 I** 1 lib %f¥^ I l%MlHk 1 1 lH 1 Ea^^l%#%l cy C S. ASHCRAFT'S ^^^^■■HB^m^HmnBEI^^HHHBV (Paten (Patent Pending) Trademark registered and copyrighted. Patent applied lor. The C. S. Ashcraft Mfg. Co., Inc. 36-32 Thirty Eighth Street, Long Island City, N. Y. It does unheard of things in screen lighting. Such as ... up to 100% increase in screen side-lighting. It works on any current from 78 to 115 amperes. It does it on a single size positive carbon, 11mm in diameter. It will exceed accepted industry standards in lighting an outdoor screen 52 feet wide. It is the answer to the ill-lighted Drive-ln screen up to 95 feet in width. It is a miserly brute in its operating expenses. Saves you up to 50% in carbon costs. Want proof? Call your National Theatre Supply Co. man. He will prove the CORE-LITE worth in indoor or drive-in theatres with existing screens and lenses. International Projectionist March 1963 11