238 BELL SYSTEM TECHNICAL JOURNAL 



ments which must be met for such a mounting are small areas, accurate 

 alignment, and adequate pressure to hold the plate in place. Some de- 

 signers make slight indentations in the quartz at the point of contact to 

 improve the mechanical stability of the plate. For crystal plates of the 

 order of one-half inch square or smaller, points having an area of about 

 10 mils in diameter are employed and the pressures used for holding the 

 plate range from one to two pounds. For larger plates correspondingly 

 larger areas of points and increased pressures are employed. The accuracy 

 of alignment required for ten mil points is of the order of two or three mils. 

 This is obtained in the mountings shown in Figure 13.1 by using con- 

 centric sleeves for holding the points which are brought into alignment by 

 means of a straight rod and then cemented in place. One of the points is 

 fixed while the other point slides in its sleeve and the pressure required is 

 obtained by the spring which presses on the outer end of the sliding point. 

 In balanced filter structures it is desirable to use crystal plates with the 

 coating on each side divided into two equal areas. This reduces to one-half 

 the number of plates that would otherwise be required. In mounting plates 

 with divided coating, it is necessary to provide a mounting which makes 

 double contact on each side of the plate. Figure 13.1 (A) shows a pressure 

 type mounting which accomplishes this. This is the mounting which has 

 been used for several years in holding the plates used for the 75-type crystal 

 channel filters^ for the standard terminal common to all broad-band tele- 

 phone systems. The crystal is mounted in the holder in such a way that 

 the two pairs of points clamp the crystal along the nodal line. The rectangu- 

 lar dimensions of the points used for this type mounting for crystals oper- 

 ating in the frequency range from 60 kc up to 120 kc are about 35 mils long 

 in the direction of the nodal line and from 10 to 15 mils wide. A very 

 important requirement for such a mounting is that the flat area of the 

 points on each side of the plate fall in the same plane. This is accomplished 

 by a precise milling operation after the points are assembled in the mounting. 

 The pressure applied to the pair of points is furnished by the flat spring 

 shown and is equalized by the action of the roller centrally located under 

 the springs. The pressure employed is of the order of four to five pounds 

 for each pair of points. 



The most commonly used coating for crystal plates held in pressure-type 

 mountings is aluminum.^ Aluminum has been found to be most satisfactory 

 for this type of unit because its hard surface is more resistant to wear at the 

 points of clamping than other metals such as silver, or gold. 



Except for a few designs, which are mounted in sealed metal or glass 



* "Crystal Channel Filters for Carrier Cable Systems," C. E. Lane, B.S.T.J., Vol. XVII, 

 Page 125. 



^ The details of processing aluminum-coated crystals are similar to those described for 

 silver-coated crystals in paragraphs 13.42 and 13.43. 



