354 



MAGNETIC METHOD 



[Chap. 8 



measurement of magnetic anomalies on a moving support. The accuracy 

 was approximate!}'' 4O7. 



Slow-speed cyclotrons^^ are designed to measure the time required for an 

 electron to make one complete loop or revolution in the earth's field. 

 Since the path of the electron adjusts itself at right angles to the direction 

 of the field, the total intensity is determined thereby. The present ac- 

 curacy is only 100 to 3OO7. Mechanically, the principle involved is 

 equivalent to a hood with two opposite windows placed over a source of 

 electrons. At a given moment one of the windows is opened and the 



(a) 



I ^^ 



I 



^v 



/o 



20 30 



Gauss 



JO 



JO 



(b) 



Fig. 8-34. Effect of axial magnetic field (a) on paths of electrons, (6) on plate current 

 in magnetron. (After Hull.) 



electrons are permitted to pass out to complete their loop in the magnetic 

 field and return to the other window. Bj^ opening this window at the 

 correct time and by providing means for recording the arriving electrons, 

 the time required for one loop may be determined. In actual operation, 

 the source of electrons is a heated wire, and the opened and closed windows 

 are suppressor grids. They are connected to a tube oscillator and are 



39 See K. H. Stehberger, Terr. Mag., 38(3), 184 (Sept., 1933); and Ann. Physik, 

 17, 278-292 (1933). 



