1888.] 



Magnetic Qualities of Nickel, 



377 



as well as in that of bromine, the magnitudes of the minimum pro- 

 portions of substance necessary to change the potential of magnesium- 

 platinum, zinc-platinum, and cadmium-platinum couples, varied 

 directly as the atomic weights of the positive metals. 



The experiments also show that the degree of sensitiveness of the 

 arrangement for detecting the minimum-point of change of poten- 

 tial depends largely upon the kind of galvanometer employed. As a 

 more sensitive galvanometer enables us to detect a change of potential 

 caused by a much smaller proportion of material ; and as the propor- 

 tion of substance capable of detection is smaller the greater the free 

 chemical energy of each of the uniting bodies, it is probable that the 

 electromotive force really begins to increase with the very smallest 

 addition of the substance, and might be detected if our means of 

 detection were sufficiently sensitive or the free chemical energy was 

 sufficiently strong. 



VII. "Magnetic Qualities of Nickel (Supplementary Paper)." 

 By J. A. EwiNG, F.R.S., Professor of Engineering in Univer- 

 sity College, Dundee. Received June 14, 1888. 



(Abstract.) 



The paper is a supplement to one with the same title by Professor 

 Ewing and Mr. GO. Cowan, which was read at a recent meeting of 

 the Society. It describes experiments, conducted under the author's 

 direction by two of his students, Mr. W. Low and Mr. D. Low, on the 

 effects of longitudinal compression on the magnetic permeability and 

 retentiveness of nickel. The results are exhibited by means of curves, 

 showing the relation which was determined between the intensity of 

 magnetisation of the metal and the magnetising force, when a nickel 

 bar, reduced to approximate endlessness by a massive iron yoke which 

 formed a magnetic connexion between its ends, was magnetised under 

 more or less stress of longitudinal compression. Corresponding curves 

 show the relation of residual magnetism to magnetising force, for 

 various amounts of stress ; and others are drawn to show the relation 

 of magnetic permeability to magnetic induction. Initial values of the 

 permeability, under very feeble magnetising forces, were also deter- 

 mined. The experiments were concluded by an examination of the 

 behaviour of nickel in magnetic fields of great strength. Magnetising 

 forces ranging from 3000 to 13,000 c.g.s. units were applied by 

 placing a short bobbin with a narrow neck made of nickel between the 

 poles of a large electromagnet, and it was found that these produced a 

 practically constant intensity of magnetisation which is to be accepted 

 as the saturation value. 



