MAGNETIC LENS WITH POLE PIECES 



341 



angle 6 with the z axis, and enters the magnetic field at B, which extends 

 around the equatorial plane OX, it will be continuously deflected until 

 it reaches the field-free space CD. From C onward it wall move in a 

 straight line to intersect the z axis at D. The trajectory is not in one 

 plane, however. It is a screw motion which at every complete turn 

 crosses this axis. For a given object distance OA, the image distance 

 OD does not depend on the angle of projection 6, if 6 is small. 



Busch [1926] and later Davisson and Calbick [1931] showed that for 

 electrons of moderate velocity the focal length / of an electron lens is 



1 0.022 



V 





H, 2 dz cm 1 



where V is the energy per electron in volts and H is the field intensity in 

 oersteds. The value of H along the z axis may be obtained from the 

 currents induced in a miniature probe coil as it is pulled out of the 

 field, from various points above and below the z axis. The resulting 

 distribution of H z is shown in Fig. VIII-196. If this curve is replotted 

 in terms of H 2 , the area under this new curve is the value of the integral 

 H 2 dz. 



Fig. VIII-20. A typical electron lens pole piece. Electron lens with pole piece 

 in place. The tier of three coils may be used interchangeably to vary the magnetic 

 field in pole-piece gap. 



Magnetic Lens with Pole Pieces 



Ruska [1934] developed an electron lens of an iron-cored type to be 

 used as an objective in his electron microscope Two common types 

 of iron pole pieces that may be inserted in the magnetic field of an 

 electron lens are shown in Fig. VIII-20. These pole pieces are about 

 30 mm in diameter and 25 mm thick, with a 10-mm gap. The lens is 

 designed so that one or two coils may be used to modify the magnetic 

 field between the pole pieces. 



In such annular magnetic pole pieces the field intensity may attain 

 3500 oersteds and will drop to 1000 oersteds in the fifth millimeter below 

 the gap. 



