Sec. 3-1] 



MAGNETIC TRA XSDl Vl-Jh'S 



193 



aa in Figs. (3-1)17 and (3-1)18, and the current I b is measured. 1 The 

 method is applicable for field strengths below 20 oersteds and for 

 small variations of field strength. The sensitivity is high, of the order 

 of 20 mA/oersted. Feedback circuits for the increase of sensitivity 

 and stability are described by Rossiger. 2 



2. The current I b is kept constant by variation of the plate volt- 

 age E b , as shown by the line bb in Fig. (3-1)18. The voltage E b is 



Fig. (3-1)17. Characteristics of a 

 magnetron illustrating the plate 

 current / as a function of the mag- 

 netic field strength B for different 

 plate voltages E b . 



Fig. (3-1)18. Plate current- 

 plate voltage characteristics of 

 a magnetron; parameter, mag- 

 netic field strength. 



measured. The method is primarily applicable for medium field 

 strength and for large variations of field strength. A direct-reading 

 instrument based upon this method is described by Gundlach. 3 The 

 range extends from about 200 to 1,200 gauss. 



3. A resistance is inserted in the plate circuit; the applied voltage 

 E b is kept constant, and the current I b or the voltage across the 

 resistor or the magnetron tube is measured, as shown by line cc in Fig. 

 (3-1)18. This method, first proposed by Hull, is applicable for small 

 or medium fields, depending upon the size of the resistor. 



The magnetron transducer has the advantage that no moving 

 parts and no variation of the measured field are required. The inertia 

 of the system is negligible up to 10 -8 sec. The probe can be made 

 fairly small and has no effect upon the measured field. Weinzierl 4 has 

 used anode cylinders with a diameter as small as 0.5 mm sealed in 

 glass tubes of 6 mm diameter. Ferromagnetic materials, such as 

 nickel, must be avoided in the construction of the tube. If operated 

 under temperature-limited conditions, the filament temperature 



1 A. W. Hull, Phys. Rev., 22, 279 (1923); M. Rossiger, Z. Physik, 43, 480 

 (1927), and Z. Instrumentenk., 49, 105 (1929). 



2 Rossiger, ibid. 



3 F. W. Gundlach, Arch. tech. Messen, V 391-3, July, 1938. 



4 P. Weinzierl, Rev. Sci. Instr., 21, 492 (1950). 



