16 



CIRCULAR MAGNETISATIONS IN IRON TUBES. 



reversed will be to impart to the tube a mean circular magnetisation of definite 

 amount. The induction will not, at reversal of the axial current, oscillate to equal 

 amounts on the opposite sides of zero induction. On the contrary, it will oscillate 

 about a mean value of definite amount, so that the total induction will be less on one 

 side than on the other. In the ballistic method of measuring inductions, however, this 

 one-sidedness or bias would not be apparent. But if the explanation suggested above be 

 the true one, the induction due to reversal of the axial current should be greater when 

 this current is also directed steadily without reversal along the tube. The experiment 

 was accordingly tried in January of 1892, not with the tubes investigated above (which 

 being in Japan were unfortunately not at my disposal), but with a short tube supplied me 

 by Professor Tait, to whom I would here express my thanks for liberty to work in the 

 Physical Laboratory of the University of Edinburgh. The tube was 10 cm. long, had an 

 external diameter of 4*1 cm., and an internal diameter of 2'7. The average field through- 

 out the region occupied by the iron wall is consequently 0122 for one ampere of current 

 along the axis. The following are some of the results obtained, the first column giving 

 the current in amperes, the second column containing numbers proportional to the induc- 

 tion produced by reversal of the axial current, and the third column similar induction 

 numbers when the current was in addition sectional but steady : — 



Current in Amperes. 



Axial Induction. 



Pure. 



With Sectional Current. 



3-26 



4-44 



843 



16-85 



29-8 



43-3 



87-3 



194-6 



30 

 43 



85-6 

 194-5 



There is no evidence here that the sectional current has any distinct effect upon the 

 susceptibility. At any rate, what effect is produced is certainly not increase. 



The discrepancy in Table III. is consequently not to be explained in terms of a direct 

 effect upon the susceptibility. We must look for the explanation in the other directions 

 indicated. That is to say, the ordinary theory of the circular magnetisation of iron under 

 the influence of an axial, or a sectional, current does not strictly apply. It will be noticed, 

 however, that the discrepancy is not large, amounting only to 7 per cent. To this 

 approximation, therefore, we may regard the ordinary theory as holding for sectional 

 currents whose current density does not exceed 0*2 ampere per square centimetre. For 

 much greater current densities, as when, for example, a fairly strong current passes along 

 a thinnish iron wire, nothing can be asserted ; and it is difficult to see how an experi- 

 mental investigation into the circular magnetisation of a solid wire could be undertaken. 



