184 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



permits a relatively simple determination of the field direction if 

 the position of the contact brushes can be varied; the output is a 

 minimum when the position of the brushes is in the direction of 

 the magnetic field. An instrument built on this basis is described 

 by Peters. 1 Its most sensitive range is 0.04 to 2 oersteds, its accuracy 

 1 to 2 per cent. 



The methods illustrated in Figs. (3-1)3 and (3-1)4 are applicable 

 for field strengths above about 10 -2 oersted. If the fields are not 



Transformer 



Flexible coupling and 

 drive mechanism 



Fig. (3-1)5. Rotating search coil with inductive coupling to the out- 

 put circuit. 



homogeneous, the output will not be purely sinusoidal and will con- 

 tain higher harmonics, so that an absolute determination of the field 

 strength is difficult. Errors are likely to arise from brush noise, con- 

 tact potentials, and thermoelectrical potentials at the brushes. The 

 use of silver or siiver-graphite brushes lubricated with a thin layer 

 of petroleum jelly has been recommended. 



A method to overcome contact errors has been developed by Wills 2 

 and is illustrated in Fig. (3-1)5. The rotating coil is driven by a 

 flexible cable; the connections from the coil are led to the primary 

 winding of a transformer which rotates within a stationary secondary 

 coil. The output from the secondary coil is measured. The loss of 

 signal level due to leakage of the magnetic field of the transformer is 

 considerable. 



Errors may arise from variation of speed of the driving motor, from 

 mechanical instability, such as oscillation of the coil and the suspen- 

 sion during rotation, and, in particular at low field strengths, from 

 stray fields from the driving motor. Distances up to two meters 

 between the motor and the pickup coil have been employed to reduce 

 this source of error. Variations with time of the measured field 



1 W. A. E. Peters, Elektrotech. Z„ 71, 193 (1950). 



2 M. S. Wills, J. Sci. Instr., 29, 374 (1952). 



