ELECTRIC power to operate auxiliaries is now an indispensable requirement of modern aircraft. Within the past ten years the electric load on an airplane has increased more than tenfold.1 The optimum voltage for electric systems for large aircraft already has more than exceeded the present 24-volt standard,2,3 and the use of higher-voltage systems, already tried experimentally,4 now appears to be a certainty. Moreover, the operating altitudes of modern airplanes, particularly military craft, are climbing steadily to ever increasing heights. Each of these factors indicates a need for a careful study of the requirements for insulation of aircraft electric apparatus in order to achieve minimum weight and space without sacrifice of reliability. A preliminary study of this subject was made and reported5 prior to the use of the 120-volt a-c system on the XB-19, but the projected use of 208-volt systems4 for still higher altitudes makes a more comprehensive study of this problem most timely.
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