190 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



is turned on or off or if the coil is removed from the two points, a 

 voltage-time integral will arise between the output terminals, which 

 is 



/ 



V 



The number of turns n, the cross-sectional area a of each turn, and 



Fig. (3-1)11. Search coil for measure- 

 ment of magnetomotive force. 



Belt 



Fig. (3-1)12. Cross section of the search 

 coil illustrated in Fig. (3-1)11. 



the length I can be measured separately, or the coil can be calibrated 

 empirically. 



3-12. Systems Based upon Magnetic Effects on Moving 

 Charges 



a. Rotating Disk. A magnetic transducer based upon the effect of 

 the magnetic field upon the electrons in a moving-solid conductor is 



illustrated in Fig. (3-1)13. It consists 

 of a rotating metal disk (copper, alumi- 

 num) with contact brushes applied to 

 the axis and the periphery. The con- 

 duction electrons in the metal disk are 

 moving with the disk in circular paths. 

 Under the influence of the magnetic 

 field, the electrons are deflected in a 

 direction perpendicular to the mechan- 

 ical movement and the magnetic field, 

 thus causing a potential difference be- 

 tween the center of the disk and the 

 periphery. With the notations of Fig. (3-1)13, the (d-c) output 

 voltage is 



e = ""(»■? — r\)fixH 



Fig. (3-1)13. Rotating disk ("uni- 

 polar" generator) as a transducer 

 for magnetic field strength. 



