Sec. 3-1] MAGNETIC TRANSDUCERS l!)l 



where/ is the frequency of rotation in revolutions per second. The 

 sensitivity of the method is low. 1 



b. Induction in Moving Conductive Liquids. Instead of a moving 

 solid conductor, a conductive liquid flowing through a magnetic field 

 can be used for the construction of a magnetoelectric transducer. 2 



Figure (3-1)14 shows a schematic 

 diagram of the mercury jet mag- 

 netic transducer of Kolin. A stream 

 of mercury passes through an insu- 

 lating tube with the average veloc- 

 ity v. Two fixed, diametrically 

 opposite electrodes A and B are 

 inserted in the tube, their small 



end Surfaces flush with the inner Fl °- (3-1)14. Mercury-jet magnetic 

 n pj.ij.1. txjx- transducer [from A. Kolin, Rev. Sci. 



walls 01 the tube. Instead ol mer- T . ,, „ L X Q /1q ^ K m 



Instr., 16, 209 (194o)]. 



cury, any other conductive liquid 



can be used (e.g., salt water). 3 If the transducer is exposed to a 

 magnetic field H in the direction perpendicular to the plane formed 

 by the tube axis and the line connecting the electrodes, a voltage 

 appears between the electrodes A and B of a magnitude 



E = dvfxH 



where d is the distance between the electrodes, i.e., the internal 

 diameter of the tube. A mathematical analysis of the transducer 

 principle is presented by Thurlemann, 4 with consideration of electri- 

 cal eddy currents originating in the conductive liquid. The output 

 voltage is proportional to the average flow velocity for both laminar 

 and turbulent flow and is independent of the velocity distribution 

 over the cross section of the tube. 



Since the mercury is flowing continuously, fast field changes can 

 be followed and the method is applicable to the measurement of time 

 varying or a-c fields of a frequency up to several kilocycles. Field 

 directions and field gradients can be measured by the use of further 

 pairs of electrodes. 



The absolute accuracy depends on the accuracy with which d, the 

 separation of the electrodes, and v can be determined. The velocity 

 can be measured with an accuracy of 0.1 per cent. With reasonable 



1 Cited by H. Neumann, Arch. tech. Messen, V 391-5, September, 1939, 

 and V 391-4, August, 1940; also L. W. McKeehan, J. Opt. Soc. Am., and 

 Rev. Sci. Instr., 19, 213 (1929). 



2 First proposed by A. Leduc, J. Physique, 46, 184 (1887). 



3 See B. Thurlemann, Helv. Physica Acta, 14, 383 (1941). 



4 Thurlemann, ibid. 



