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MANAGERS AND PROJECTIONISTS 501
mits of very efficient ventilation, and decreases the weight of material required for a unit of given output.
The machine may be had for 25, 33, 40, 50 and 60 cycles, and either for single, two or three phases in the same frame. The machine is now standardized, and obtainable in the following sizes, viz.: double 50 ampere, which is adjustable from 40 to 60 amperes ; double 75 ampere, adjustable from 60 to 85 amperes ; and double 120 amperes, adjustable from 75 to 135 amperes, all three types operating either one arc or two arcs in series.
With this type the wiring is simple, there being required one loop from one terminal of the generator through the first projector switch, then through the second projector switch.
and through a third, if it is a three projector installation, the switch of the last projector being connected to the other terminal of the generator.
The diagram. Fig. 175, illustrates the very simple connecFigure 174. tions between the
motor generator,
the instrument panel, and the projector table switches, it being understood that if three projectors are installed, it merely means extending the loop of the circuit to include an additional projector table switch.
The over-all dimensions of the Hallberg Series Arc Motor Generator referred to is thirty inches in length, by about twenty-two inches diameter for the largest size. The weight varies from 450 to 650 pounds, depending upon the capacity of the set, and the phase for which the machine is made, the heaviest being the 120 ampere capacity machine, wound for 25 cycles, single phase current.
Some theatre managers insist upon complete duplicate motor generator installation. For such a demand the Hallberg motor generator is made with different winding, so as to provide the maximum number of amperes required for the operation of one arc only, and then two motor generators are installed as per Fig. 176, connected to a set of transfer