462 On Magnetic Induction in Iron and other Metals. 



Recurring to the heat analogy, let us imagine the terminal 

 portion of a very long conducting bar to be subjected to time- 

 periodic change of temperature. When the periodic changes 

 have become steady throughout the bar, let there be plotted 

 the graphs of time-temperature for two sections of the bar 

 whose distances from the end of the bar are, say, x x and &v 

 In dealing numerically with the data obtained from the ex- 

 periment, however, we can only use the value (x 2 — #1), 

 because practically the source of the changes of temperature 

 operates not merely at the section x = but over a finite 

 region in its neighbourhood. Conversely, if all the other 

 data were known and we had to calculate the values of x for 

 the two sections of the bar, we could only expect to find an 

 approximate agreement between the observed and the cal- 

 culated values of x. The larger the value of x, the more 

 close would be the approximation. 



In the corresponding magnetic problem, my attention was 

 directed to these considerations by the partial failure of a 

 line of attack different from that of this paper, on the problem 

 of determining constants. 



How the question really stands is a matter for further re- 

 search. Much light could be thrown on the whole subject 

 by a systematic series of experiments on a material, such as 

 an iron wire, by subjecting it to a cyclical variation of H of 

 constant period whilst it is kept under a strain which is 

 altered step by step. To make this clear, we might for 

 example have 



H varying 0, + 90, 0, — 90, whilst the load on the wire 



is zero ; 

 H varying 0, -h 90, 0, —90, whilst the load on the wire 



is 1 kilog. ; 

 H varying 0, -f 90, 0, —90., whilst the load on the wire 

 is 2 kilogs. ; and so on. 



A similar systematic series of experiments could be made 

 with different temperatures, instead o£ with different loads, 

 for the same period of H. 



I feel sure that the information obtained by the harmonic 

 analysis of the results of such experiments would well repay 

 the expenditure of the time and energy involved. 



These suggestions I would take the liberty of recommend- 

 ing to the favourable consideration of the Advisory Com- 

 mittee of the National Physical Laboratory, in connexion 

 with their proposal to experiment on the hysteresis of iron 

 and its alloys. 



Gordon's College, Aberdeen, 

 Auo-ust 1901. 



