98 



G. Barns — Apparent Hysteresis. 



If, however, the breakdown is instantaneous, due for instance 

 to the molecular shake-up accompanying magnetization or its 

 withdrawal, then viscosity is instantaneously a minimum, the 

 deformation sudden and the phenomenon " static." There 

 seems to be no theoretical gap here. 



2. The method of work is the same as that employed in 

 preceding experiments.* The two identical wires to be 

 compared, ab, cd< are fastened coaxially one above the other 

 and a mirror, m, is attached between them. The top and the 

 bottom of the system are soldered to cor- 

 responding torsion heads, D, E, and the 

 wires are insulated from each other at the 

 mirror. The lower wire is kept submerged 

 in a tube of flowing water, ww, and the 

 meansf are at hand for passing an electric 

 current through it without interfering 

 with the torsional adjustment. Hence the 

 lower wire is under the influence of a cir- 

 cular magnetic field and thermal discrep- 

 ancies are reduced to a minimum. Again 

 a helix, AA, is placed around the tube for 

 a longitudinal field the water bath being 

 retained. 



As the two identical wires make a single 

 system any degree of twist may be locked 

 up in it and the change dn of rigidity n 

 due to the magnetic field is dn/n = d6/6, 

 nearly, where 6 is the twist imparted and 

 dO the change of twist due to the action 

 of the field and observed at the mirror. 



In describing the experiments L will 



denote the length of each wire in centims., 



diameter in centims., C the current 



D their common 

 either through the lower wire or through the helix in amperes. 

 3. The effect to be observed expresses itself in a shifting of 

 the fiducial zero, i. e., the position of the spot of light on the 

 scale when the magnetic field is nil. The amount of deflec- 

 tion obtained in the first and second magnetizations therefore 

 differs because the zero is markedly changed and in the case of 

 a circular field always in such a way as to diminish the deflec- 

 tion ; in longitudinal fields on the contrary, in a way to 

 increase the deflection. 



*This Journal, [4] x, pp. 407-418, 1900. 



f The vanes v v dipping in the salt solution contained in an annular trough (not 

 shown), deaden vibrations and carry off the electric current for the circular field, 

 entering at E. The weight W suitably stretches the system with the aid of a 

 slot adjustment in the torsion head E. 



