338 COMPOUND AND ELECTRON MICROSCOPES 



can either diverge or converge the beam. Figure VIII-17a will give an 

 image of the cathode in the plane of the arrow at the right and can be 

 used to produce a magnified image of the surface structure of the cathode. 

 Figure VIII-176 will give an image of the cross-over area located just 

 to the left of i while the image of the cathode is located at i. 



Owing to the non-uniformity of emission, it is not desirable to use 

 the cathode as the electron source. It is more desirable to use an inter- 

 vening aperture as an electron source since it can be made smaller and 

 denser and also allows some latitude of focusing. This is taken advan- 

 tage of in George's [1929] design of an electron gun (Fig. VIII-17c). 



The beam from the cathode C is concentrated and brought to a focus 

 at the hole in plate P by properly proportioning the spacing between 

 filament C, filament shield S, and plate P with respect to the size of the 

 aperture S. The high-potential field is so shaped, by the anode cylinder, 

 that the electrons are not only accelerated in the direction of the anode 

 but are also given radial components of velocity which crowd them to 

 the axis of the beam. If this radial force component compensates the 

 repulsive force of the beam's space charge it is possible to bring the 

 beam to a sharp focus. If the diameter of the beam is wide, where it 

 starts to converge, the electron gun will have a long focal length. By 

 increasing the radial component, it is possible to change the focal 

 length of the beam. This is done by shifting the anode A by means of 

 the knurled head H which operates a rack and pinion. 



The Magnetic-Field Electron Lens 



If a moving electron, with its associated magnetic field, enters a 

 static magnetic field it may be deflected so as to change the curvature of 

 its path. The static magnetic field does not change the kinetic energy 

 of the electron. If the electron moves in a direction perpendicular to a 

 homogeneous magnetic field, it is deflected to describe a circular path. 

 If it moves parallel to the direction of the field, it is altered neither in 

 direction nor in speed. If the electron makes an angle with the direc- 

 tion of the magnetic field it will corkscrew down the field as indicated 

 in Fig. VIII-18. This complex motion is more easily understood after 

 considering the motion of an electron which is shot, with velocity v, 

 into a homogeneous static magnetic field of intensity H set at right 

 angles to the direction of motion of the electron. This magnetic field 

 forces the electron to move in a circular path. 



The static magnetic field acts at right angles to the magnetic field 

 associated with the moving electron. The electron is therefore acted 

 upon by a radial force F r = —evH. While moving in this circular 

 path the electron is subjected to a centrifugal force —F c = mv 2 /r. In 



