192 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



physical dimensions, the sensitivity is of the order of 0.4 microvolt/ 

 oersted. The instrument is applicable to the measurement of field 

 strengths above 1 oersted. 



c. Magnetron Tubes. A magnetron, shown in Fig. (3-1)15, con- 

 sists of an electron-emitting cathode C surrounded concentrically by 

 a cylindrical anode A . Electrons move from the cathode to the anode 

 under the action of an applied voltage E b and form a current / in the 

 external circuit. The path of the electrons is radial, as shown in 



Fig. (3-1)15. Magnetron tube, sche- 

 matic diagram and circuit. 



Fig. (3-1)16. 



Electron paths 

 in a magnetron. 



Fig. (3-1)16, path 1. If a magnetic field H is applied in the direction 

 of the tube axis, the electrons will travel in a curved path (path 2), 

 and if the field strength is increased, a condition will be reached 

 in which no electrons reach the anode (path 3) and the current 

 diminishes to zero. The condition at which this occurs depends upon 

 the intensity of the magnetic field H and the applied plate voltage 

 E b . The critical magnetic field strength at which current cutoff occurs is 





(r 2 cl r l)] 



(1) 



where e is the charge and m the mass of the electron, r c the radius 

 of the cathode, and r a the radius of the anode. If the diameter of the 

 cathode is small compared to that of the anode, the term 1 — (r^/t'l) 

 in Eq. (1) can be neglected. 



In practice, the critical magnetic field strength is not accurately 

 defined. The current in Fig. (3-1)17 decreases with a finite slope 

 owing to tube asymmetries, variations of the initial velocity of the 

 electrons, space charge (occurring in particular at low plate voltages), 

 fringe effects at the ends of the cylinder, and, in directly heated tubes, 

 differences of the effective plate voltage along the axis. Empirical 

 calibration of the transducer is required for accurate results. 



Three basic methods for the operation of the magnetron transducer 

 are available: 



1 . The plate voltage E b is kept constant, as indicated by the lines 



