International projectionist (Jan-Dec 1948)

Switches: Projection's Tremendous Trifle BECAUSE the fundamental purpose of a switch is to perform the apparently simple task of completing and interrupting an electric circuit, it might at first thought seem that any switch, once installed and tested, could be taken for granted and its constructional details forgotten. Not infrequently, however, something goes wrong, and the importance and individuality of that lowly device compel our attention. i Projectionists are not concerned with switches as complex as those that open 100,000-volt power lines under full load, and which may involve a formidable system of motor-controlled relays; but it would be a serious mistake to assume that all the numerous switches in a projection installation are such that deterioration is a negligible factor, or that breakdown never occurs. Switches are the "nerve centers" of the projection setup, hence their functioning is, or should be, a matter of vital importance to the projectionist. Knife switches are the simplest and sturdiest of all types. A hand-operated knife switch of good manufacture may function for many years without readjustments of any kind or even cleaning, though such extreme neglect is an everpresent invitation to trouble. With the exception of the quick-break variety, knife switches contain no springs, pistons, or delicately hinged pivots — the weakest features of toggle switches. Relay Switches Complicated Toggle and snap switches often leave much to be desired in the way of substantial construction and firmness of the electrical contact. The ampere ratings and various seals of approval imprinted on the cheaper toggle switches mean little or nothing at the present time, the electrical trade being flooded with in Motor starting switch: sometimes used for starting d-c motors of large capacity. The jaws may be connected to any type of starting resistor, the steps being successively shortcircuited as the switch is closed. By ROBERT A. MITCHELL ferior merchandise. Projectionists in small towns can play safe by ordering all electrical items from the theatre supply houses which serve their territory. Relay switches of the type used for starting motor-generator sets and controlling auditorium lights, footlights, curtains, etc., are more complicated in that they involve an extra circuit and are opened and closed magnetically, but in general they are ruggedly built and rarely give trouble. Dirt and wear have a telling effect on all mechanical devices, however, and in one instance, at least, a generator was dismantled for overhauling when the real cause of the trouble was a faulty contact in the 3-phase relay switch. By fits and starts the motor of the motorgenerator set ran on only two phases! The switch was located in an out-of-theway corner of the cellar of the theatre, and not a few projectionists had come and gone without ever having checked its condition. At this point attention is directed to the danger, sometimes ignored, of working on "live" switches which carry high voltages. A 220-volt shock is usually little more than an unpleasantly severe jolt, but occasionally someone "freezes" to the conductors and gets killed. A shock of 550 volts is fatal more often than not. Eliminate unnecessary risks by pulling the 3-phase service switch before working on generator or rectifier relays. To make doubly certain that the relay is "dead," touch the prods of your test-light to each set of contacts. Cheap Switches Troublesome Fader switches of the relay type are frequently a source of annoyance. Although it is true that they hardly ever wear out, microscopic accumulations of carbon dust and other dirt on their contact points may cause noisy sound, crosstalk, and even sound outages. The points may be cleaned by drawing a strip of clean writing paper between them. There is a wide gulf between a generator relay and a sound fader, indeed, but the difference serves to remind us that each type of switch is specifically designed for the job it has to do. Trouble is most frequently experienced with ordinary toggle switches, especially with those which have been unwisely purchased at a dime store or salvaged from a scrap heap of discarded parts. Numberless instances are on .record of vital switches literally falling apart during show hours. It is difficult indeed to comprehend the complacent attitude of those who rely on switches that are scarcely fit even for emergency use. A variable rheostat, like a switch, is a current-limiting device and is also subject to breakdown. On my first day in a certain theatre all went well until I attempted to increase the generator voltage to its proper value by means of the field rheostat in the generator control box. The arc snapped out very suddenly and the voltage dropped to zero. The rheostat wiper arm had popped completely out of its bearing! A jumper fitted with battery clips (which I had carried about with me for several years without ever having used it in an emergency) promptly restored the show. A 4-foot length of insulated flexible wire having a storage-battery clip attached to each end is a mighty handy thing for any projectionist to have around. It is quite all right to make the best of what you have in an emergency, but it is exceedingly risky to replace a worn switch by one of the wrong type. This sometimes happens through pure carelessness. For example, replacement of a motor-generator starter with a standard 2-way (3-way) switch may result in burning out the relay solenoids. Most starters provide electrical contact only during the time that the button is actually depressed by the finger. Specific Switch for Specific Job Incredible though it seems, the wrong type of switch may literally wreck a projector. The following is a case in point: Trouble was experienced during a matinee with the motor switch on one of the projectors, the projectionist finding it necessary to flip the switch several times to start the motor. After the show (Continued on page 30) Quick-break knife switch: contact blade of this single-pole, single-throw switch is held by the clamping action of the jaws until the handle compresses the spring sufficiently to drive the blade out. The blade leaves the jaws with greater rapidity than in an ordinary knife switch, hence burning of the metal by the arc is minimized. 16 INTERNATIONAL PROJECTIONIST • October 1948