418 /Scientific Proceedings, Royal Dublin Society. 



Magnetizing circuits. — In the final arrangement three sections of the 

 solenoid were used to give the longitudinal field, another section to com- 

 pensate the vertical component of the Earth's field, and two others for the 

 demagnetizing circuit. 



Demagnetization. — This was carried out by reversing the direct current 

 from a 20-volt storage battery by reversal with a hand commutator at t,he 

 rate of one reversal per second, reducing the current at a uniform rate, 

 the whole process occupying two minutes and a half. This is the method 

 recommended by Burrows. 1 Eecently Smith 2 has drawn attention to the 

 fact that even this rate may be too great for complete demagnetization ; but 

 in the present set of experiments the rate of one cyele per second proved to 

 be ample, as shown either by tests with the magnetometer or observations on 

 the Wiedemann effect. 



Elimination of Hysteresis. — The elimination of hysteresis in magnitude 

 and direction under a spiral field in the first series of observations was 

 accomplished by the method used by Steinhaus and Gumlich 3 in their 

 experiments on the magnetization of iron in the absence of hysteresis. A 

 given magnetic field was applied to the solenoid, and an additional longitudinal 

 field of one cycle per second was applied and gradually decreased to zero. 

 This has been shown to give an IS curve similar to the hysteresis-free curve 

 given by tapping soft iron, and bearing a similar relation to the hysteresis 

 loop. 4 



This type of magnetization has been termed anhysteretic, which usage 

 will be adopted, in this paper. The process is a symmetrical one when 

 applied to a simple longitudinal field, but it is not so symmetrical when super- 

 imposed on a spiral field. However, it is easy to demonstrate that it gives 

 here also the required result. It has long been known that the Wiedemann 

 effect gives a hysteresis loop if the longitudinal field is kept constant and 

 the circular field (or current passing along the central wire) is varied. 



A hysteresis loop obtained in this manner is shown in fig. 1, using a 

 longitudinal field of 8 units, and varying the circular field between - 15 and 

 + 15 c.g.s. units. The anhysteretic curve was obtained by setting the circular 

 and Longitudinal fields, and then superimposing on these fields a longitudinal 

 field which was slowly decreased to zero, with reversals at the rate of one 



1 Burrows, Bull. Bureau of Stand., vol. iv, p. 205, 1908. 



2 Smith, Phys. Rev., Sept., 1917. 



3 Steinhaus and Gumlich, Ber. d. Deut. Phys. Ges. 17, 369, 1915. 



4 Ewing, Magnetic Induction, pp. 116 and 322. 



