352 Dr. Hopkinson and Messrs. Wilson and Lydall. [Apr. 20, 



April 20, 1893. 



The LORD KELVIN, D.C.L., LL.D., President, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



The following Papers were read : — 



I. Magnetic Viscosity." By J. Hopkinson, D.Sc, F.R.S., 

 E. Wilson, and F. Lydall. Received March 8, 1893. 



The following experiments were carried out in the Siemens 

 Laboratory, King's College, London, and are a continuation of ex- 

 periments by J. Hopkinson and B. Hopkinson, a description of which 

 appeared in the ' Electrician,' September 9, 1892. 



In that paper determinations were given of curves showing the 

 relation between the induction and the magnetising force, for rings of 

 fine wire of soft iron and steel, through complete cycles with varying 

 amplitudes of magnetising force, both with the ordinary ballistic 

 method and with alternating currents of a frequency up to 125 

 complete periods per second. It was shown that if the induction was 

 moderate in amount, for example, 3000 or 4000, the two curves closely 

 agreed ; but, if the induction was considerable, for example, 16,000, the 

 curves differed somewhat, particularly in that part of the curve 

 preceding the maximum induction. The difference was greater with 

 steel than with soft iron. 



It was not then determined whether this difference was a true time 

 effect or was in some way due to the ballistic galvanometer. The 

 present paper is addressed to settling this point. 



The ring to which the following experiments refer is of hard steel 

 containing about 0'6 per cent, of carbon, in the form of wire in. 

 diameter, varnished with shellac to ensure insulation. The material 

 was supplied by Messrs. Richard Johnson. The ring is about 9 cm. 

 diameter, and has a sectional area of L08 sq. cm. ; it is wound with 

 200 turns of copper wire, and with 80 turns of fine wire for use with 

 the ballistic galvanometer. 



In the 4 Electrician ' paper the static curve of hysteresis was 

 determined by the ballistic galvanometer, the connexions being made 

 according to the diagram in fig. 1 : where R is the hard steel wire 

 ring, B is the ballistic galvanometer, Sj is a reversing switch, and S 3 

 is a small short-circuiting switch for the purpose of suddenly insert- 



