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BELL SYSTEM TECHNICAL JOURNAL 



optic axis, but for practical purposes a material such as pyrex glass 

 or fused quartz is entirely satisfactory. 



The crystal holder is constructed so that a slight variation can be 

 made in the total spacing between electrodes. Since the frequency 

 varies with electrode spacing this can be used for making a slight 

 adjustment of frequency. The thread on the adjustable plate is 

 kept tight by spring tension to prevent the spacing from varying 

 irregularly. A crystal holder and a 100,000-cycle crystal are shown 

 in Fig. 7. 



Even though the crystal and its mounting have very low tempera- 

 ture coefficients, it is desirable to control their temperature, for which 



Fig. 7 — Crystal mounting with crystal. 



purpose the temperature controlling device shown in Fig. 8 has been 

 constructed. It consists of a cylindrical aluminum shell with a wall 

 about one inch thick, with a heater (not shown), and with a tempera- 

 ture responsive element in the wall to control the rate of heating. 

 The aluminum shell has a metal plug that screws into the open end 

 forming a chamber for the crystal which is then completely closed 

 except for a small hole for electrical connections. 



Since aluminum is a good thermal conductor the shell equalizes 

 the temperature throughout the chamber and thus avoids the use of a 

 fluid bath. The main heating coil is wound in a single layer over the 

 whole curved surface of the aluminum cylinder, being separated from 

 it only by the necessary electrical insulation. Auxiliary heating coils 

 are wound also on the ends so as to distribute the heating as uniformly 

 as possible. This, in effect, makes the short cylinder behave like a 



