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BELL SYSTEM TECHNICAL JOURNAL 



Huxmeter dcHection is proportional to the change in B — II. The 

 drum is conveniently rotated by an electric motor, connected by geans 

 so that the drum makes about one revolution in two minutes, and it is 

 desirable to have this rate variable. The motor may be reversed, so 

 that complete hysteresis loojis may be recorded. 



14,800 



-14,800 



Fig. 5 — Hysteresis loop of hard iron. 



In setting up the apparatus the photo-electric cell may be con- 

 veniently placed above or below the drum, and one lamp above the 

 other. The lamp used to illuminate the photo-electric cell should 

 furnish a brilliant beam, and it was found that a 250 watt Mazda 

 projection lamp was quite satisfactory. 



Fig. 6 — Hysteresis loops of hard iron, with increasing maxinmni fields. 



Calibration of the Circuit 

 The circuit is calibrated by passing a known current through the 

 primary of a known mutual inductance, the secondary of which is 

 connected in series with the search coil S. By measurement of the 



