340 



COMPOUND AND ELECTRON MICROSCOPES 



will be imaged in the plane B by the point P 2 ' . Thus a small area AA 

 emitting a stream of electrons with uniform velocity will form a point- 

 for-point image in the plane B at &B. 



The analysis of the motion of an electron in a short symmetrically 

 distorted magnetic field is beset with mathematical difficulties beyond 

 the scope of this book. An approximate graphical analysis, therefore, 

 will be used to show how a short magnetic electron lens functions. 



A short magnetic electron lens is a coil in which the region of magnetic 

 field distribution, along its axis, is small compared with its focal length. 



// 



/ / 



\\\ \ i , 1 1 ' 



* i-t ■>-( -ft -f-t u - 



' ' I i ! I I V\ 



(6) 



Fig. VIII-19. (a) Vertical section through a magnetic field of an annular sole- 

 noid enclosed in a 2-mm soft-iron shield. Condenser electron lens without pole 

 pieces, (b) Curve showing magnetic field strength in oersteds measured along Z, 

 above and below horizontal plane through A', (c) Path of an electron leaving A and 

 passing through such a magnetic field, crosses Z axis at D. 



In order to compress the field, the coil is encased in soft iron shields 

 about 2 mm thick with an inner annular gap at one end. This air gap 

 is usually made equal to about one twentieth the diameter of the inside 

 coil. Such a lens produces an axially symmetrical magnetic field, the 

 intensity of which drops off very rapidly above and below the gap. 

 Figure VIII-19 shows such a coil of 6500 turns of 0.2-mm-diameter 

 wire excited at 220 volts, 0.1 ampere current. It has a 4-mra annular 

 gap. If a probe is moved along the vertical axis, to determine the in- 

 tensity of the magnetic field, it will be found that the magnetic intensity 

 H z drops to half value at about 3 mm above and below the horizontal 

 plane of the gap. 



If a high-speed electron from a field-free space above A approaches 

 this coil and moves down the z axis, it will pass through the magnetic 

 field without changing its speed or direction. If the electron leaves the 

 point A, however, and moves along the straight line AB making a small 



