318 



Mr. J. Hopkinson. 



[June 14, 



temperature coefficient did not deviate far from 0*0388 per degree 

 centigrade of its resistance at 20° C. ; I was unable to detect that 

 the coefficient deviated from this value in either direction. The 

 temperature may therefore be taken as approximately accurate. 



The nickel had the form of a ring — fig. 1. On this ring was 

 wound in one layer 83 convolutions of No. 27 B.W.G. copper wire 

 carefully insulated with asbestos paper to serve as measurer of 

 temperature and as secondary or exploring coil. Over this again, a 

 layer of asbestos paper intervening, was wound a coil of 276 con- 

 volutions in five layers of No. 19 B.W.G. copper wire to serve as the 

 primary coil. 



The method of experiment was simply to pass a known current 

 through the primary, to reverse the same and observe the kick on a 

 ballistic galvanometer due to the current induced in the secondary. 

 At intervals the secondary was disconnected, and its resistance was 

 ascertained for a determination of temperature. Knowing the 

 current it is easy to calculate the magnetising force, and knowing the 

 constants of the galvanometer it is easy to calculate the induction 

 per square centimetre. The practice was to begin by heating the 

 ring to a temperature at which it ceased to be magnetic, then to 

 lower the gas flame to a certain extent and allow the apparatus to 

 stand for some time, half an hour or more, to allow the temperature 

 to become steady, then determine the temperature, then rapidly make 

 a series of observations with ascending force, lastly, determine the 

 temperature again. The ring was next demagnetised by a series of 

 reversals with diminishing currents. The flame was further lowered, 

 and a second series of experiments was made. It was then assumed 

 that the previous magnetisation would have a very small effect on 

 any subsequent experiment. As the substance turned out to be far 

 from pure nickel, it is not thought worth while to give actual 

 readings. The results are given in the accompanying curves, Nos. 1 

 to 14, in which the abscissas represent the magnetising forces per 

 linear centimetre, the ordinates the induction per square centimetre, 

 both in C.G.S. units. Curves 15 and 16 give the results of Professor 

 Rowland* for pure nickel at the two temperatures at which he 

 experimented. In curves 17 to 20 are given the inductions in terms 

 of the temperature for stated intensities of the magnetising force, 

 the ordinates being the inductions, the abscissas the temperatures. 



An inspection of these curves reveals the following facts : — 



1. In my impure nickel much greater magnetising forces are 

 required to produce the same induction than are required in Professor 

 Rowland's pure nickel. 



2. The portion of the curve which is concave upwards in my 

 sample is less extensive and less marked than in his. 



* ' Phil. Mag.,' November 1874. 



