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MANAGERS AND PROJECTIONISTS 495
of both, as regards distance from collector lens and angle of crater, be the same, and the optical train be identical, the screen illumination from each must and will have precisely the same value.
Another plan which has merit is that of the motor generator having a 70-volt dynamo using ballast resistance to reduce the generator voltage to arc voltage. The waste involved is very slight and the plan has its advantages.
D. C. to D. C. SETS. — There is a special motor generator set made, known as the "D. C. to D. C. motor generator," which operates, in effect, merely to reduce the voltage. (See Page 521.)
GENERAL INSTRUCTIONS ON MOTOR GENERATORS. — In the interest of economy of space we shall give certain instructions which apply alike to all motor generators. To incorporate these in the matter covering each individual machine would merely be a reiteration of the same thing several times.
WARNING. — Exhibitors often complain to the author that their motor generator set is not as efficient as was claimed by the manufacturer. This may very easily be true without any fault on the part of the maker, for while a machine in perfect condition may show high efficiency, after a few months of unintelligent handling, or abuse, it may show something very different. Factory efficiency tests are made with machines in the very pink of condition. They naturally show very much higher than after a few months under the care of a careless or slovenly attendant, or one who knows very little and perhaps cares less beyond how to start and stop them, and to put fresh oil in the oil wells "once in a while" Loose connections, dirty brushes and roughened commutators do not make for efficiency.
It occurs, too, that the projectionist will permit his art, which should operate at a certain length and voltage, to open to a length considerably greater with probably 10 volts more drop. This, on a constant current generator, means an increase of current consumption of about 15 per cent, and a corresponding drop in efficiency without any gain on the screen.
GENERAL INSTRUCTIONS NO. 1, LOCATION.— Several things must be given very careful attention when the location for a motor generator is considered.
If it be practical it is decidedly better to locate the motor generator in a room directly adjoining and connecting with the projection room. If this is not a practical thing to do, then it may even be located within the projection room itself.
A basement location is, for several reasons, objectionable, and if the basement be damp and dark it should not be considered at all. Where there is dampness, the insulation of the wires will absorb more or less moisture while the machine is idle. This moisture will be expelled rapidly when