272 BELL SYSTEM TECHNICAL JOURNAL 



the other hand, the use of a relatively long etch, 30 minutes or more, is 

 desirable when a high Q is desired. The long etch also results in an im- 

 proved stability of the resonant frequency as a function of current. This 

 will be discussed in a subsequent paragraph. A disadvantage of a long 

 etch is the difhculty of controlling the etching process within close toler- 

 ances. The variations in rate of removing material may be sufiScient to 

 affect the uniformity of the linear dimensions of the plates. 



These factors indicate that etching is an important process in preparing 

 crystal plates. A close control must be maintained on the strength of the 

 acid, the uniformity with which the surfaces of the plates are exposed and 

 the duration of the exposure. 



14.3 The Effects of Deviations during Fabrication of Wire- 

 Supported Unit 



As described in Chapter XIII'", two types of mountings have been de- 

 veloped for supporting crystal plates, the Pressure Type and the Wire- 

 Supported Type. The wire-supported type of mounting is the more recent 

 development and has resulted in crystal units which have a much higher 

 degree of stability and can be reproduced within much closer tolerances 

 than the units using the pressure type of mounting. Since this chapter is 

 concerned chiefly with the problem of obtaining a high degree of precision 

 in crystal units, the discussion is restricted to the wire-supported type of 

 mounting. 



14.31 Silver Spotting 



For the wire-supported type of mounting the first operation is to bake 

 small silver spots on the surface of the crystal plates. In the application 

 of these silver spots to the crystal plates three factors are of importance in 

 their effect on the characteristics of the plate, namely, the size of the spot, 

 its location, and the firing temperature. Since in all crystal designs to 

 date the silver spots are applied at or near the nodal line of the crystal plate 

 the principal effect of the spots is to increase the stiffness of the plate, so 

 shghtly increasing the frequency of resonance. Variations of an appreciable 

 magnitude in either the amount of silver paste used (that is, the size of the 

 spot) or in the location of the spot with respect to the nodal line will change 

 the resonant frequency of the plate. Such changes could be corrected 

 later, when the plates are adjusted for resonant frequency, as long as the 

 length is increased sufficiently to allow such adjustment. However, if the 

 length be increased sufficiently to allow for extreme cases, avirage adjusting 

 time will be increased materially, while if the allowance is insufficient some 

 of the plates may be unusable. For this reason, close control of the size 

 and location of the silver spots is well justified. 



