ETCH TECHNIQUE 49 



plate are of both opposite sense of cut and opposite electrical sense, as may 

 be observed from the third and fourth views of Fig. 5.4. The effect of this 

 difference in sense of cut for the two types of twinning is brought out by 

 Table I, which gives the approximate frequency constants and temperature 

 coefficients for the common cuts of oscillator plates, together with those for 

 the analogous, oppositely (and hence wrong) sensed cuts. 



In the case of a CT plate, for example, both portions of an optically twinned 

 plate (cut at +38°) will be of the same +38° orientation. The plate is 

 elastically the same throughout and hence should exhibit the frequency and 

 low temperature-coefficient desired. However, the opposed electrical 

 senses of the two portions will cause a reduction in the electrical activity. 

 The amount of this reduction will depend upon the relative size of the two 

 portions and upon their placement relative to the vibration nodes of the 

 plate. 



On the other hand, when a CT plate is electrically twinned one portion of 

 the plate will be of the correct +38° orientation while the other portion is of 

 the incorrect —38° orientation. The two portions of the plate have widely 

 different elastic properties, as is exhibited in the table by the different fre- 

 quency constants and their respective temperature-coefficients. Resulting 

 from this difference alone, the plate will exhibit operating characteristics 

 (if operable at all) intermediate between the two listed in the table (usually 

 near one of these two), and its activity will be reduced. The activity will 

 also be reduced by the opposite electrical senses in the two portions. The 

 degree to which the frequency, temperature-coefficient, and activity are 

 affected, again depends upon the relative sizes of the two portions of the plate 

 and their placement relative to the ''nodes" of the plate. 



Thus, for equivalent proportions and placement of twinning, electrical 

 twinning will cause a much greater change in the operating characteristics 

 of the plate than will optical twinning.-^ 



A note may be inserted regarding the electrical testing of plates, some of 

 which may be twinned while others may be untwinned but of incorrect sense 

 of cut. As seen from Table I, untwinned plates of the correct sense of cut 

 are easily distinguished from those of the incorrect sense of cut by their 

 frequency. This distinction between sense of cut holds as well for plates 

 containing very little twinning. The presence of appreciable twinning in 

 the plate is easily distmguished b\- the activity of the plate. While ordinar- 

 ily a plate would be electrically tested in the mode of vibration it is intended 

 to be operated in, it is sometimes of advantage to test it in a different mode. 



-5 In the case of the uncommon ■"combined-twinning" the two portions of the plate are 

 of opposite sense of cut but of the same electrical sense. The effect on the operating char- 

 acteristics will be like that for electrical twinning, except that the activity may not be as 

 greatly reduced. 



