194 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



must be kept very constant to keep the tube from drifting. At high 

 field strengths and high plate voltages the operation tends to become 

 unstable and oscillations of very high frequencies may set in. 



d. Electron-beam Magnetometer. The observation of the magnetic 

 deflection of a long, focused electron beam furnishes a sensitive 

 method for the detection and measurement of very small magnetic 

 fields and field variations in the order of 10~ 6 oersted. 1 



Electron 

 gun assembly 



Co/lector 

 plotes 



Differential 

 amplifier 



Collector 

 plates 



Fig. (3-1)19. Electron-beam magnetometer, schematic diagram. 



The arrangement is shown schematically in Fig. (3-1)19. It con- 

 sists of a vacuum tube with an electron-gun assembly (electron- 

 emitting, accelerating, and focusing system) at one end. At the other 

 end at a distance of 1 m are two semicircular collector plates. The 

 collector plates are overlapping but electrically separated. An elec- 

 tron beam of small cross section strikes both plates, and if the beam 

 is symmetrical, the electron currents to both plates are equal. The 

 current is measured by a differential d-c amplifier; the difference 

 furnishes a measure of the beam deflection caused by a magnetic 

 field H, in Fig. (3-1)19 assumed to be perpendicular to the drawing 

 plane, and hence can be used for the determination of the field. 



The deflection of the electron beam is 



y = 0A5kBl 2 ~= 



Ve 



where y is measured in centimeters; B in gauss; I, the length of the 

 beam, in centimeters; and E, the accelerating voltage, in volts. The 

 constant k is determined by the optics of the system (order of magni- 

 tude 0.5 to 0.7). 



1 L. Marton, L. B. Lederer, and J. W. Coleman, Natl. Bur. Standards Rept. 

 2381, March, 1952. 



