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July, 1944
MILITARY AMPLIFIER DESIGN
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breathing of moisture, but there is also drying out and consequent loss of capacitance. In modern military construction paper capacitors are either of the oil-impregnated, molded type, not wax impregnated as are their civilian prototypes, or else are of the oilimpregnated, oil-filled, metal-cased type. Fig. 3 shows an ordinary commercial paper tubular capacitor and its metal-cased "bathtub" equivalent. As a quick check as to whether or not capacitors are properly sealed, samples are placed alternately in baths of warm and cold water. If they are improperly sealed, the capacitors will either short circuit, or there will be a marked decrease in the insulation resistance.
Solar Mfg. Photo
FIG. 3. This photo shows an 0.5 tf, 600-v paper capacitor of the type used in ordinary commercial amplifiers, left, and a hermetically sealed, metal-cased capacitor of the same rating designed for military electronic equipment.
-As far as the electrolytic capacitors are concerned, they are considered a necessary evil to be avoided whenever possible. In some cases, of course, it is not possible to avoid their use. Then special electrolytics, properly designed to withstand the specified operating ambients, are used. But, as you know, electrolytics designed for low-temperature operation are not the best design for high-temperature applications and vice versa. Since motion picture amplifiers are not ordinarily used at temperatures below freezing you may think hightemperature electrolytics would be all right; but it must be remembered that equipment may be stored in tin shacks on deserts or transported in ships' holds stacked next to steam pipes.