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494 HANDBOOK OF PROJECTION FOR
arc to produce results very nearly equal to the direct current arc. This, however, does not hold good to any considerable extent, and broadly speaking does not in the least invalidate our former statement.
The Projection Department of Moving Picture World and the author of this work unqualifiedly recommend the installation of either a motor generator set, or a mercury arc rectifier, with the notation that modern practice favors the motor generator set as against the mercury arc rectifier, because of the fact that the motor generator is a very much more flexible machine. The motor generator may be temporarily overloaded by as much as 100 per cent., though, of course, such an overload could only be maintained for a moment or two — sufficient time, however, for change-over — whereas the mercury arc rectifier cannot be overloaded to any considerable extent ; also the motor generator may be had in any desired capacity, whereas the mercury arc rectifier is not ma'de in anything exceeding 50 amperes capacity.
Let it be clearly understood that we do not recommend the overloading of a motor generator by 100 per cent., even for a short time, but such a machine should carry a 50 or even a 75 per cent, overload for as much as three minutes, with reasonable frequency, without sustaining injury.
MOTOR GENERATORS.— In the ordinary acceptance of the term as applied to projection, the motor generator is nothing more or less than an alternating current motor, of suitable voltage, cycle and phase to operate on the available supply, direct coupled to a direct current dynamo, the latter, in latest and best practice, being of the constant current type, i. e., wound to deliver an approximately steady amperage at considerable variation in arc voltage.
In fact the latter phase of the matter has been carried to such an extent that arc voltage may be doubled without appreciably altering the amperage. The latest practice is the motor generator, the dynamo of which is so wound that when it is "pulling" one projection arc, a second can be cut in series with the first, whereupon the voltage of the dynamo doubles, the amperage remaining constant.
The advantage of this type is that with a minimum effort on the part of the projectionist the change-over may be made without the slightest evidence of the act on the screen. This is by reason of the fact that with the arcs operating in series, in the very nature of things both of them must and will have precisely the same amperage, and if the adjustment