200 



Prof. J. A. Ewing and Mr. W. Low. [Mar. 24, 



so that the lines of force at the disk are normal to it, but the direction 

 of the force is reversed as we cross the axis of y. The current-function 

 relatively to axes displaced through the proper angle tj in the direc- 

 tion of rotation, varies as 



2/v/{l-r 2 /« 3 }. 



A drawing of the current lines for this case is given. As already 

 mentioned, they are simply the orthogonal projections of the contour 

 lines of the tessaral harmonic of the second order. 



In the next type we have n = 3, s = 2, so that 



and the current-function, relatively to displaced axes as before, 

 varies as 



IV. " On the Magnetisation of Iron in Strong Fields." By Pro- 

 fessor J. A. EwiNG, B.Sc, F.R.S.E., University College, 

 Dundee, and Mr. WILLIAM Low. Communicated by Sir 

 W. Thomson, Knt., LL.D., F.RS. Received March 2, 

 1887. 



(Plate 2.) 



The behaviour of iron and steel when subjected to very strong 

 magnetising forces is a matter of considerable practical and very 

 great theoretical interest, especially from its bearing on the molecular 

 theory of magnetisation, which assigns an upper limit to the intensity 

 of magnetism that a piece of iron can acquire, and even suggests that 

 the metal may become diamagnetic under the influence of a suffi- 

 ciently great force. All experiments hitherto made, by magnetising 

 iron in the field of an electric solenoid, have shown that the intensity 

 of magnetism JJ, as well as the induction is increasing with the 

 highest values actually given to the magnetising force J. It is 

 scarcely practicable, however, to produce by the direct action of a 

 magnetising solenoid, a field whose force exceeds a few hundreds of 

 C.Gr.S. units. 



To refer to a few recent experiments of this class : — In experiments 

 by one of us* on the magnetisation of long wires, the highest value 

 of || applied to iron was about 90, and this gave an induction Jjjj of 

 16,500 in a soft iron wire. In Dr. Hopkinson's experiments^ a force 



* Ewing, " Exp. Kes. in Magnetism," ' Phil. Trans.,' 1885, Part II. 



f J. Hopkinson, " Magnetisation of Iron," ' Phil. Trans.,' 1885, Part II. 



