A HIGH PRECISION STANDARD OF FREQUENCY 



499 



section from an infinite cylinder. To protect the thermostat from 

 the effect of ambient temperature gradients the heating coil has an 

 outside covering consisting of four layers each of thin felt and sheet 



Fig. 8 — Temperature control chamber with crystal mounting. 



copper spirally wound so that alternate layers are of copper and felt, 

 the innermost layer being of felt and the outer one of copper. This is 

 the covering that appears on the complete device shown in Fig. 9. 



Fig. 9 — Complete temperature control unit. 



This covering is very effective in reducing surface gradients since the 

 conductivity in directions parallel to, and perpendicular to, the 

 surface differ by a large ratio. 

 33 



