A HIGH PRECISION STANDARD OF FREQUENCY 



495 



which may vary with time. A form of crystal is desired which can be 

 mounted so that the parts vibrating at relatively large amplitude do 

 not bear heavily on any portion of the mounting. 



■^v.' 



Fig. 2 — Partly assembled crystal showing the relation of the slab to the crystal axes. 



Further study of temperature coefficients showed that the rings 

 remaining after the small discs had been cut from the larger one, 



A, +0.0045 



Ba +0.0037 



A3 +0.0004 



B I +0.0047 A 2 +0.0033 B3 +0.0024 



Fig. 3 — Temperature coefficients of some discs and rings. 



shown in Fig. 1, have a temperature coefficient lower than discs of 

 the same diameter and thickness. This is further illustrated in Fig. 3 

 which gives the temperature coefficient of two discs and the four 



