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BELL SYSTEM TECHNICAL JOURNAL 



the collector would be excessively small and that this leakage current, 

 unless guarded against, might prove an intolerable disturbance. 



The lead wire from the inner box is guarded from the frame of the 

 apparatus at all points of support within the tube by electrodes con- 

 nected with the guard cylinder. This lead wire and the wire from the 

 guard electrodes leave the tube through remote seals as indicated in 

 Fig. 3. The isolation of the latter of these seals was a matter of con- 

 venience rather than of precaution. 



Four electrical connections are required to parts of the movable 

 system — two to the filament and one each to the collector and to the 

 guard electrodes. These are maintained, with the exception of that to 

 the collector, through platinum tipped molybdenum brushes which 

 bear upon platinum rings. The connection to the collector is through 

 a flexible spiral of tungsten wire lying in the axis of rotation. 



Outside view of the experimental apparatus. 



It may be well to add to this description of the tube a few words in 

 regard to the adjustment of the reflectors to their proper positions and 

 orientations. Each reflector is attached, as has been already men- 

 tioned, to a triangular frame which is supported through three ad- 

 justing screws from the diagonal wall of the enclosure. By turning 

 these screws the reflector can be rotated through small angles about 

 any axis parallel to the wall, and by the same means its distance from 

 the wall can be varied. These adjustments were sufficient for locating 

 the beam reflected from the first mirror in the axis of rotation, and that 

 reflected from the second in the axis of the collector. They were not 

 sufficient, however, for meeting the further requirement that the 

 incident beam should lie in a {111 }-azimuth of the crystal structure. 

 For this adjustment we relied upon orienting the reflector correctly 

 with respect to the triangular frame at the time of its attachment. A 

 mosaic of sharply defined triangular etch pits was visible under the 

 microscope on the surface of the crystal reflector, and it was only 

 necessary to relate these properly to the triangle formed by the frame 



