Sec. 3-1] 



MAGN E TIC TRA S'S Dl'c E RS 



203 



d. " Peaking Strips." The arrangement is shown schematically in 

 Fig. (3-1)27. It consists of a wire probe M of a ferromagnetic ma- 

 terial, such as permalloy, which exhibits a hysteresis loop, as shown 

 in Fig. (3-1)28. The wire probe is surrounded by three coils. In Fig. 

 (3-1)27 these are shown separately; in practice the coils should be 

 long and overlapping. The coil C 1 is connected to an a-c source and 

 produces an alternating magnetic field varying periodically between 

 —H s and -\-H s . Every time the field passes in the positive direction 



Fig. (3-1)27. "Peaking-strip" magnetic 

 transducer. 



Fig. (3-1)28. HB characteristic 

 of the peaking strip. 



through -\-H Q and in the negative direction through —H , the mag- 

 netization changes sharply and causes a positive or negative pulse 

 in the second coil 2 . These pulses can be detected by means of a 

 cathode-ray oscilloscope. If the a-c source which supplies the coil C x 

 is also applied to the X-axis deflection system of the oscilloscope, the 

 pattern will be symmetric. If now an external magnetic (d-c) field is 

 applied, the two peaks appear asymmetric until the effect of the 

 external field upon the wire probe is compensated for by means of a d-c 

 current applied to the coil C 3 . The magnetic field strength produced 

 by the coil 3 is equal and of opposite direction to the external field; 

 the instrument does not require an empirical calibration. 



The instrument is useful for the measurement of magnetic fields 

 in a range from 0. 1 to above 500 oersteds. The upper limit is set by 

 the maximum field strength attainable in the coil 3 . If this coil C 3 is 

 water-cooled, an upper limit of 5,000 oersteds may be reached. The 

 smallest deviations which may be detected are of the order of 0.02 

 oersted. The limitation is due to thermal effects and to mechanical 

 instability of the system. The output is, in general, of the order of a 

 few millivolts; the output impedance 10 to several hundred ohms. 



