192 



BELL SYSTEM TECUXICAL JOURXAL 



each point on the cathode there is a corresponding point on the image. Tlie 

 sharpness of the image may be increased by increasing the strength of the 

 magnetic field and the field required for any degree of focus is not sharply 

 critical. 



Figure 2 shows a series of drawings of the various electron images that 

 were obtained as the magnetic field-strength was increased in a tube having 



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340 dcLuss 



Fig. 2. — Drawings of the patterns obtained with a fluorescent coating on the inside 

 of the anode when the magnetic field strength is increased from zero to the focus values. 



a fluorescent coating on the inside cylinder. The cathode and anode diam- 

 eters were 0.0625 and 2.5 inches, respectively, and the axial length was 2 

 inches. The anode was held at 150 volts. Only one-half inch of the cathode 

 length, located centrally along the axis, was coated to emit electrons. The 

 image at 340 gauss appeared to be one-half inch long. In attempting to 

 interpret these patterns it should be remembered that on the two sides of 

 the cathode at right angles to the plane of the beam the electrons follow 



