414 BELL SYSTEM TECHNICAL JOURNAL 



ening of the air-gaps. When, as usual, several materials are used in the 

 construction, bending of the stator plates due to strains introduced by 

 unequal expansions of the members produce unpredictable changes in 

 capacitance. This is particularly true in the most common construc- 

 tion where the stators are held in place by rods of insulating material. 

 The insulator having a different temperature coefficient of expansion 

 than the plates, the difference in the expansion causes the plates to 

 buckle. 



Better stability can be obtained in a condenser built as follows. All 

 parts determining the length of the condenser, including the stator 

 supports and the stator plates are of aluminum. The ends of the 

 stator supports are held in place by insulating bushings of sufficiently 

 small dimensions to make the difference in expansion negligible. The 

 bushings are made of Alsimag, a ceramic material which has a small 

 dielectric constant and coefficient of dielectric constant. 



With such a construction, the ternperature coefficient of the con- 

 denser is equal to twice the temperature coefficient of expansion of the 

 material of which the rotor plates are made minus the temperature 

 coefficient of linear expansion of aluminum determining the length of 

 the air-gaps. One half of the rotor plates are made of invar and one 

 half of aluminum. The average expansion of the area of the rotor 

 plates equals then the temperature coefficient of linear expansion of 

 aluminum and the temperature coefficient of capacitance of the con- 

 denser should be equal to the temperature coefficient of air dielectric 

 constant which is about 1 ppm per °F. negative. Measurements show 

 that the temperature coefficient of the condenser varies from — 3 to 

 -f 4 ppm per °F., a quite acceptable value. The capacitance change 

 due to a variation in the atmospheric pressure of one inch, a large 

 variation, is 20 ppm. 



Temperature coefficients, of paraffined mica condensers, can be ad- 

 justed by special manufacturing methods to 10 ppm negative. For the 

 sake of increasing the instantaneous stability the two condensers used 

 in each oscillator are paired within 3 ppm. As mentioned before, no 

 residual error due to A Co remains after the frequency check is made. 

 The low temperature coefficients are desirable only to improve the 

 stability of the oscillator. 



By using high Q circuits and suitable corrective reactances, the 

 variations in the frequency due to power line variations may be readily 

 kept smaller than any one of the other errors discussed above. 



Scale Errors 

 A heterodyne oscillator cannot be classified as a purely electrical 

 circuit for it is used to translate a mechanical coordinate, the scale 



