XXI 



A paper " On the Theory of Alternating Currents " (1884) discusses 

 the action of one alternate current machine on another when the two 

 are connected in parallel or in series. It shows that the machines will 

 not work together in series, for they control each other's phase so as to 

 nullify each other's effects ; but that they will work together in parallel, 

 the mutual control being then such as to produce synchronism. This 

 conclusion was verified by experiments on a pair of alternate current 

 dynamos intended for use at the lighthouse of Tino. Incidentally the 

 same paper touches on the theory of induction coils, a subject which 

 Hopkinson developed in a later paper.* There, in remarkably brief 

 compass, a complete theory is stated of the action of the closed-circuit 

 transformer. Later still, Hopkinson returned to the discussion of the 

 alternate-current dynamo, and described in a paper written jointly with 

 Mr. E. Wilson, f a series of experiments undertaken to examine the 

 currents induced in the coils and cores of the field magnets by the 

 movement and variation of currents in the armature. 



His earliest important paper dealing with the magnetic quality of 

 metals was published in 1885,} under the title "Magnetisation of 

 Iron," and it is characteristic of the modesty of the man that in 

 the preamble, after speaking of the work of other experimenters in the 

 same field, he says, " I have had great doubts whether it was desirable 

 that I should publish my own experiments at all." In point of fact, 

 the paper is conspicuously valuable. It contains many useful data 

 relating to samples of steel of known and very various composition, as 

 to magnetic permeability, magnetic hysteresis, and electric resistance. 

 One of the samples tested was the curious alloy or mixture of steel and 

 manganese invented by Mr. Hadfield, which is almost wholly destitute 

 of magnetic quality. The magnetic measurements were made by a 

 novel method, each sample being a short bar, which was brought 

 approximately to the condition of endlessness, in the magnetic sense, 

 by being enclosed within a massive yoke of soft iron. Apart from its 

 originality in respect of both experimental method and results the 

 paper contains much suggestive comment on points of theory connected 

 with the experiments. It came to be regarded, quickly and rightly, as 

 a landmark in the development of the subject. 



An examination of the magnetic properties of nickel at various 

 temperatures followed. This showed that in the specimen of not 

 very pure nickel tested the magnetic quality was lost when the tem- 

 perature rose to about 310 C. But the loss was somewhat gradual 

 over a range of some 50, and observations of the rate of cooling 

 from a high temperature showed no sudden liberation of heat, such 



* ' Roy. Soc. Proe.,' 1887. 

 f 'Phil. Trans.,' A, 1895. 

 J ' Phil. Trans.' 

 ' Roy. Soc. Proc.,' 1888. 



