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304 RICHARDSON'S BLUEBOOK OF PROJECTION
enough to maintain the amperage without change for a limited period every time a second light source is cut into operation in series with the first, thus doubling the load. The parallel type generator is a "constant voltage" and the series type a "constant current" machine.
(92) The design of motors and generators has been greatly altered and improved since motion pictures first came into use. Since their participation in projection, motor generators have undergone many marked alterations as the requirements of the projection room changed.
In the early days of projection, the arc light source was served through rheostatic resistance from both a. c. and d. c. supply lines but it was recognized from the first as a wasteful procedure. Various forms of choke coil were substituted for resistance, because they were far more economical than the rheostat, though more or less erratic in operation, and the light obtained from current through a choke coil had a very harsh, "blue" tint that was not pleasing.
Finally, along about 1907 J. C. Hallberg, a motion picture supply dealer, developed a low-voltage transformer especially designed for use with projection arcs. It yielded a definite saving over rheostatic resistance and delivered a decidedly better projection light source current than the choke coil. Since its cost was reasonable it was very generally adopted for use with a. c. projection arcs. This transformer, known as the "Hallberg Economizer," was the first important step in the control of current for projection light sources.
The superiority of the d. c. arc as a projection light source was so obvious, that soon there was a widespread demand for a practical means of converting a. c. into d. c, which brought about the development of the mercury arc rectifier, the rotary converter, the motor generator set and other devices, including the electrolytic rectifier. In the beginning all these machines were /xude, but they were rapidly improved, as competition became keen to capture the waiting theatre market.
The motor-generator was recognized as a superior machine because it was very flexible in the amount of