326 Profs. Dewar and Fleming on Electrical Resistance 



and the axis of 23 represent the self-demagnetizing forces due 

 to the cuts at all stages of the magnetizing process. Curves 

 giving the applied force *£)' in relation to 23 for the cut ring 

 may then be drawn by using as abscissae the distances of 

 points on the original curves from the line OPQ, measured 

 parallel to the axis of «£). The reverse of this construction 

 will of course serve to deduce true magnetization curves (for 

 uncut metal) from experiments made with a magnetic circuit 

 which contains a joint or a gap. 



The figure is drawn to scale for an actual case, and serves 

 to show that the division of a ring 30 cm. long into two half 

 rings abutting against each other with the smoothest possible 

 joints has the effect of reducing the residual magnetism from 

 9000 to 6000, which is the height of the point P. A similar 

 shearing over of the curves, with consequent reduction of the 

 residual magnetism occurs, to a greater or less extent, in 

 magnetic tests of bars when these are made to form part of a 

 complete magnetic circuit by the addition of a massive iron 

 yoke. 



XXXVII. On the Electrical Resistance of Pure Metals, Alloys, 

 and Non-metals at the Boiling-point of Oxygen. By James 

 Dewae, LL.D., P.R.S., Professor of Chemistry in the Royal 

 Institution, fyc, and J. A. Fleming, M.A., D.Sc, F.R.S., 

 Professor of Electrical Technology in University College, 

 London, $*c* 



§ 1. OBVERAL observers have studied the behaviour of 

 k3 metals as regards electrical conductivity at low 

 temperatures. In particular MM. Cailletet and JBouty 

 (Journ. de Physiqve, July 1885) have made observations of 

 the resistance and resistance-change with temperature of 

 various metals at —100° C. by the employment of liquid 

 ethylene as a cooling agent. Wroblewski (Cornptes Renclus, 

 1885, vol. ci. p. 161) measured the electrical resistances of 

 wires of electrolytic copper at various temperatures, 100° C, 

 20° C, 0° C, -100° C, and gives also figures for the com- 

 parative resistance of the same wires at the critical point of 

 nitrogen, the boiling-point of nitrogen, and the temperature 

 of the solidification of nitrogen. The possession of means for 

 producing very considerable quantities of liquid oxygen as 

 well as liquid ethylene has placed at our disposal an opportu- 

 nity of carrying out some investigations on the comparative 

 electrical resistance of a number of pure metals, alloys, and 



* Communicated by the Authors. 



