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BELL SYSTEM TECHNICAL JOURNAL 



enter the inner box of a double Faraday collector and a galvanometer 

 of high sensitivity is used to measure the current to which they give 

 rise. An appropriate retarding potential between the parts of the 

 collector excludes from the inner box all but full speed electrons. 



Fig. 2 — Showing the three principal azimuths 



The collector may be moved over an arc of a circle in the plane of 

 the drawing as indicated, and the crystal may be rotated about an 

 axis which coincides with the axis of the incident beam of elec- 

 trons. Thus the collector may be set for measuring the intensity of 

 scattering in any direction relative to the crystal — by turning the 

 crystal to the desired azimuth, and moving the collector to the desired 

 colatitude. The whole solid angle in front of the crystal may be 

 thus explored with the exception of the region within twenty degrees 

 of the incident beam. 



Certain of the azimuths related most simply to the crystal structure 

 we shall refer to as "principal azimuths." Thus there are the three 

 azimuths that include the apexes of the triangle. If we tind the 

 intensity of scattering depending on colatitude in a certain way in 

 one of these azimuths, we expect, of course, to find it depending upon 

 colatitude in the same way in each of the other two. We shall call 

 these the " A-azimuths." On the left in Fig. 2 the crystal has been 

 turned to bring one of the A-azimuths into the plane of rotation of 

 the collector. 



Another triad of principal azimuths consists of the three which 

 include the mid-points of the sides of the triangle. These we shall 

 call the "B-azimuths." The next most important family of azimuths 

 comprises those which are parallel to the sides of the triangle; of 

 these there are six, the "C-azimuths." 



