182 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



by calibrating the coil in a magnetic field of known field strength, 

 such as the inside of a long induction coil. An accuracy of 0.05 per 

 cent can be obtained. 



The search coil method is applicable for fields from 10~ 3 oersted up 

 to the highest available field strengths. For fields of a strength less 

 than 10 -3 oersted, the output voltage is usually too small for con- 

 venient measurement. 



a-c fields. If the field strength varies sinusoidally with time, the 

 induced voltage is sinusoidal too and can be measured, either directly 

 or after amplification, with a-c instruments. The method is also 

 applicable to fields of periodic but not sinusoidal character if the 

 necessary corrections for wave shape are applied. The sensitivity 

 of the a-c magnetic field strength measurement can be very high. 

 Fields of the order of 10 -5 oersted have been measured. 1 Stray 

 magnetic fields, mostly of line frequency but also of higher harmonics, 

 can introduce considerable error. The error can be reduced by mag- 

 netic shielding of the source of the stray field or eliminated by placing 

 the axis of the pickup coil at an angle of 90° with respect to the stray 

 field. 2 It is also possible, though experimentally difficult, to elimi- 

 nate the influence of the stray field 

 by compensation with a second 

 field of proper magnitude, phase, 

 and wave shape produced by an 

 auxiliary coil. 



For low frequency of the a-c 

 field, the output from the coil is 

 frequently too low for convenient 

 detection. At high frequencies in 

 the rf range, the capacitive coup- 

 ling between adjacent turns of the 

 pickup coil tends to reduce the output. 



b. Moving Coils. When the magnetic field strength cannot be 

 varied, the induction method can be applied by changing the coil 

 position in such a way that the total flux linked with the coil changes 

 by a defined amount. A simple arrangement consists of a coil with 

 flexible leads, Fig. (3-1)2, which is moved from a region where the 

 field strength is practically zero into the magnetic field ( AO = O) or 

 in the opposite direction (AO = — O). The output voltage-time 

 integral is observed. This method is particularly applicable for the 

 measurement of fields in narrow gaps, using flat coils. Instead of 



1 R. Bernard and F. Davoine, Compt. rend. acad. sci., 231, 6S7 (1950). 

 2 K. S. Lion, Funk, 5, 431 (1928). 



Fig. (3-1)2. Moving search coil. 



