The film daily year book of motion pictures (1932)

best results he should allow from 25 to 30 volts for ballast loss. It is the .part of wisdom to arrange means for the projectionist to watch the arc voltage. This is provided for in many of the larger houses by having a voltmeter wired into the arc circuit of each lamp and located near the lamp, generally on the wall of the projection room in easy sight at all times. One manufacturer of generators provides a switch on his panel so that the projectionist can readily get the arc voltage of any piece of apparatus. This requires the running of an additional No. 14 wire to each lamp circuit at a point between the arc and its ballast. It is bad practice to put the ballast into the same room with the motor generator sets. The amount of heat liberated by the ballast at full load is generally as great or greater than that given off by the generator itself, and unless a room is extraordinarily well ventilated, such heat contributes to the general overheating of the room and of the generator. It must be understood that high temperatures in and about a generator set have nothing to recommend them from any standpoint. By high temperatures we mean from 90° Fahrenheit up. It is true, too, that the owner or projectionist is often unnecessarily alarmed about the generator heating, for on a hot day he may lay his hand on the frame or some other metallic part of the unit and it will feel uncomfortably hot. He should remember that his hand when moist, as it will be in warm weather, will burn when laid on metal at a temperature of 120° Fahrenheit. With a permissible temperature rise of 72° Fahrenheit this would presuppose the room at 48° Fahrenheit, which, of course, is unreasonable, except in winter if the room is unheated. So reasoning from the other end, a summer temperature of 100° in the room and a 72° rise would bring the metal temperature to 172° which the hand would not endure for an instant but which is to be expected from generators with a standard rating as to heating. The generator room should preferably be ventilated to the outdoors, two openings being provided. In this way there is no added heat to the house and whatever noise is present does not become an additional element of annoyance. The setting of a generator unit is of importance, and generally speaking, each manufacturer has his recommendations as to how best to accomplish it. The important thing in setting a unit is not to fasten it down with bolts or screws because. if only fairly in balance, it will not move even if not held down at all. It is important to so place it as to not have vibrations transmitted into the floor which supports it. The older type of vertical units were easier to set than the present horizontal ones because the heaviest vibration was at right angles to the length of the shaft so that in a vertical machine a fair amount of cork or rubber underneath would suffice nicely because since the vibration of the body of the machine was in a horizontal plane it carried less into the supporting floor. With a horizontal machine the requirements are more severe and more care is necessary. One of the most common mistakes is to make the support too rigid. For instance, the cork used is selected of too solid or hard a kind and too large a sheet is used so that the degree of flexibility is not sufficient to kill the vibration. Cork, if elastic and properly applied, is good. A high quality of rubber while possibly not as durable as cork is probably better, having greater elasticity than cork. Spiral springs are often used and are excellent. A very inexpensive and efficient plan is to place the unit on a pair of planks themselves longer than the unit and supported from the floor at their ends. These planks must be chosen so that the weight of the unit produces a fair degree of deflection. It is always well to bear in mind that where the run from the generator to the projection machines is long the voltage loss should be checked into, rather than to depend entirely upon Underwriters' rules to determine the wire sizes. If the drop in such lines is considerable, a dimming of the picture will result in changeover similar to that which occurs when a generator has an excessive voltage drop under the same conditions, and it will accentuate whatever generator voltage drop there is. Where possible it is better, and sometimes cheaper, to run separate lines from the generator room to the various arcs. When this is done the Underwriters' rules can be adhered to. At all events it is well in every case to use wire plenty large enough because conditions may change so that heavier currents may be needed later. Above all, use conduits of ample size. A small additional initial outlay for such material will result in a safer and more readily installed job and will permit the use of a larger wire, or additional wires, later should they be required. THE FILM DAILY CIRCULATES IN EVERY COUNTRY (J 985