International projectionist (Jan-Dec 1950)

From a Manufacturer's Casebook Differences of opinion anent fundamental technical law, no less than that applicable to horse races, make possible the presentation of this down-to-earth and informative data relative to an everyday problem of practical projection procedure. Comment on this article is invited. SOME of the most interesting technical data of a highly practical nature relating to daily operating problems reposes in the files of equipment manufacturers and rarely is made available for the information and guidance of the projectionist craft generally. These data are in the form of letters which have been exchanged between manufacturers, on one hand, and supply dealers and projectionists on the other. Always in the forefront of those enterprising manufacturers who stand ready at all times to render maximum service to the field, the Strong Electric Corp., of Toledo, Ohio, maker of arc lamps and other projection equipment, has made available to IP a file of letters bearing on a specific projection problem. Except for the deletion of the dealer's name and that of the theater, these letters are presented here verbatim. To Strong Electric Corp. We received two Strong Suprex rheostats for the . . . theater. These rheostats were ordered for 120-volt d. c. current, within the 40-70 amps range for Suprex carbons. On installing these rheostats, we are unable to pull more than 60 amps per lamp. I immediately checked the d. c. voltage coming into the projection room and found that it measured 115-116 volts. Now, the problem is this: could these rheostats have been a 60-amp type which you shipped in error? We want to pull about 68 amps. The old rheostats used in this theater were pulling 60 amps, but an increase in light is desired now. Reply To Dealer All you need do to increase the arc current range is to cut off the resistance wire and then reconnect the wires to the bus bar at the bottom and back ends of the rheostats. These rheostats as shipped are correct for normal conditions. However, in some instances when the theater wiring may be small or the runs are long, so that there is an abnormal high voltage dip in the wiring, there may be too much resistance wire in the rheostat. These rheostats are designed so that by simply cutting off some of the resistance wire they may be adjusted to any local condition. To shorten the resistance wire, turn the rheostat upside down, cut off the coiled resistance wire by about 4 inches, loosen the bolts on the copper bus bar and throw away all the short ends of the coils. Then stretch the remaining coils along their entire length and reconnect the ends to the bus bars. It is a simple job. To Strong Electric Corp. As previously explained, there is no appreciable drop in voltage in this particular projection room. We took a voltmeter and got voltage readings both at the panel and at the projector. The readings were 115 volts at both spots. We cannot expect the theater to do the work making the changes in your rheostats as per your instructions, nor do we feel that we should go to the trouble and expense involved in this job. Please advise your preference: whether you wish the work done at your expense or shall we return the rheostats? 'Appreciable Working Margin While you are about it, perhaps it would be better to supply rheostats which would deliver up to 80 amps, rather than Using the diameter of a 7-mm carbon as an accurate gauge for determining correct arc gap length of 5/16 inch. to 70, so that we would have an appreciable working margin. It seems to me that if we cut off portions of the coils, as you suggest, we are going to alter the voltage we get at the arc, thus embarassing us further. I have not taken the time to work this problem out by the use of Ohm's law. Reply To Dealer Your recent letter indicates that you are somewhat confused regarding your Ohm's law and the behavior of a highintensity arc, with the result that you attach all the blame to the rheostats. These rheostats are exactly as you ordered them, that is, they are adjustable from 40 to 80 amperes when the arc gap length is correct at 5/16 inch and the line voltage, as measured when the arc is burning, is 110 volts. At 115 volts these rheostats are adjustable up to about 84 volts. However, if the arc gap be too long or the line voltage too low, the normal result will be low arc amperage. We shall try to explain how to correct for either of these abnormal conditions, which obviously are no fault of the equipment. Two Adjustments Necessary To increase the amperage of a highintensity arc it is always necessary to make two separate adjustments: the rehostat is turned up and then the carbon feed on the lamp must be increased to compensate for the faster burning of the carbons at the higher amperage, so as to maintain the same 5/16-inch arc gap length. To turn the rheostat up without speeding up the carbon feed will simply jump the arc amperage for a few seconds; but as soon as the current rises the carbons burn faster, the arc gap naturally lengthens and causes the amperage to drop back to just where it was previously, and one can keep on increasing the rheostat until one is blue in the face without getting the desired result. The rule covering this point is simple: when the rheostat is turned up the carbons must be fed faster; failing this, all one gets is the same amperage and a longer arc gap length, not increased arc current. The proper arc gap length at 65 amps 12 INTERNATIONAL PROJECTIONIST • MARCH 1950