Professor J. A, Ewing 



[May 22, 



230 



convenient means of practically testing the quality of iron in this 

 respect tlie lecturer had lately introduced another instrument, which 

 was also shown at work. In this hysteresis tester (Fig. 5) the 

 sample of iron, in the form of a bundle of thin strips, was clamped 

 in a carrier and caused to rotate between the poles of a magnet 

 swinging on knife edges. As a consequence of hysteresis this 

 magnet was deflected, and its deflection, which was noted by means 

 of a pointer and scale, served to measure the hysteresis. 



Fig. 5. 



To show directly the heating effect of magnetic reversals in iron, 

 a differential air thermometer was used, with long bulbs, one of 

 which was partly filled by a bundle of iron wire. Both bulbs were 

 surrounded by coils, through which an alternating current passed. 

 The heating effect of the current itself was the same for both, but 

 the bulb containing the iron was further heated in consequence of 

 the hysteresis of the metal, and this additional heating was shown 

 by movement of a liquid index in a tube connecting the two bulbs. 

 It had even been proposed to apply the heating effect of hysteresis 



