500 BELL SYSTEM TECHNICAL JOURNAL 



The temperature of the shell rises and falls periodically by about 

 0.02° C. but even this variation is prevented from reaching the crystal 

 in its mounting by a layer of felt about half a centimeter thick sur- 

 rounding the crystal holder. At the period of thermostat operation 

 obtained the temperature variations actually reaching the crystal are 

 reduced more than a thousand-fold. The complete temperature con- 

 trolling device is shown mounted in its operating position in Fig. 9. 

 One of the mounted crystals wrapped in its felt protecting layer is 

 shown in Fig. 10, 



To protect the resonator from humidity and pressure variations it 

 is kept under a bell jar at a pressure slightly below atmospheric. 



Fig. 10 — Crystal mounting with felt insulation. 



With the crystals used the frequency varies approximately one part 

 in a million for 10 cm. of mercury change in pressure. It is aimed, 

 therefore, to maintain the pressure constant to about ± 1 mm. A 

 small mercury gauge within the bell jar indicates the pressure, which 

 may be adjusted by a vacuum pump through a valve in the surface 

 plate. The pressure within the bell jar is affected somewhat by the 

 temperature, and in order to keep it within the required limits it is 

 necessary to maintain a rough control of the temperature within the 

 jar. The pressure gauge does not indicate a change in pressure due 

 to a change in temperature but will indicate any slow leak into the 

 jar that may develop. A thermometer within the bell jar indicates 

 the temperature, from which the change of pressure, and the correction 

 of frequency due to it, may be computed if desired. 



