EVOLUTION OF QUARTZ CRYSTAL CLOCK 



549 



source of instability, as mentioned previously in relation to the use of the 

 first ring crystals. Plating of crystals is not a new idea but the appHcation 

 to quartz resonators of high Q requires a great amount of technical skill in 

 order to obtain coatings which are mechanically and chemically stable and 

 which utilize the minimum of added material. The use of too much metal 



Fig. 21 — Wire-supported GT crystal sealed in a glass envelope. 



will, of course, impair the resonator by increasing its rate of energy dissipa- 

 tion and probably its aging rate. The metal most often used for electrodes 

 is silver, although gold and aluminum have been used in special cases. Evap- 

 oration in vacuum has been found to be the most satisfactory method for 

 the actual plating, giving very adherent coatings and being subject to 

 precise manufacturing control. The art of plating quartz resonators is 

 discussed in detail by H. W. Weinhart and H. G. Wehe in Mr. Heising's 

 book. 



