. The Bell System technical journal. Telecommunication; Electric engineering; Communication; Electronics; Science; Technology. ELECTRICAL MEASUREMENT 73 of the bridge are such that the value of each capacitance may be determined in terms of the frequency and the two Fig. l^Bridge circuit for measuring capacitance in terms of resistance and frequency The requirements for a capacitance standard are high constancy with variations in frequency, time, voltage, and atmospheric condi- tions, and a small phase difference. Mica has been found to be the best solid dielectric, used either a
. The Bell System technical journal. Telecommunication; Electric engineering; Communication; Electronics; Science; Technology. ELECTRICAL MEASUREMENT 73 of the bridge are such that the value of each capacitance may be determined in terms of the frequency and the two Fig. l^Bridge circuit for measuring capacitance in terms of resistance and frequency The requirements for a capacitance standard are high constancy with variations in frequency, time, voltage, and atmospheric condi- tions, and a small phase difference. Mica has been found to be the best solid dielectric, used either alone or impregnated with a high quality wax such as paraffin. If mica alone is used, the condenser must be sealed to prevent the entrance of moisture. Good mica condensers can be obtained with a temperature coeffi- cient below per cent per deg. cent., and having a variation of less than per cent over a frequency range from 500 cycles to 100 kc. Variations in capacitance with voltage are also negligible provided voltages below 100 volts are used. It has been our experi- ence that the paraffin-impregnated condensers generally have a nega- tive change of capacitance with temperature. This change is smaller than that of the unimpregnated type which has a positive change with temperature. The paraffin-impregnated condensers, however, usually change more with time than the unimpregnated condensers. Air condensers may be used as standards in small sizes. For the larger values, the air condensers become large and cumbersome and are not as stable as the mica condensers. Even in the smaller sizes, very special precautions must be taken to obtain air condensers which have appreciably smaller phase differences than the mica condensers, which may be made with phase dififerences considerably less than one minute. Inductance. Requirements for inductance standards are high con-. Please note that these images are extracted from scanned page images that may have been digitally enhanced
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