The Magnetic. Properties of almost pure Iron. 375 



mately calculated in our case. Take the curve in fig. 1 — we see B 

 increases 57,50, whilst H increases 6. The total change is 9860, 

 since the cross sectional area of the specimen is 1*715 sq. cm. H = 6 

 corresponds to a change of 0*117 ampere, and taking the volt as our 

 unit and the primary turns at 50, we find L = 4'21 x 10 -2 , where L 

 is the coefficient of self -ind action. If E be the applied potential and 

 R the resistance of the circuit, the current at any time t after closing 



the circuit can be expressed by ^ (1 — e - jf). For t = ^ second the 



It 



current has its maximum value within an exceedingly small quantity. 

 Bat the method of experiment enabled one to test the rapidity with 

 which the current rises to its maximum value. Let a balance be 

 made, say when the current is such that the force is equal to the 

 coercive force of the material. Now suddenly reverse the force from 

 its maximum through zero to this value. The immediate depression 

 of a key tells at once if the current is still balanced. The current 

 immediately after reversal, that is to say within \ second, had cer- 

 tainly attained its normal value to within 0*3 or 0*4 per cent. 



The condenser had no material effect upon the rapidity with which 

 the current attained its maximum value. We can only conclude 

 that the effect is peculiar to the iron itself, and might be influenced 

 by induced currents, since the ring is not subdivided. The subject 

 of propagation of magnetism as affected by induced currents has 

 been dealt with in the case of a magnet having a core 12 inches 

 diameter,* and a magnet having a diameter of 4 inches. f Imagine 

 that the cross-section of our pure iron specimen is circular instead of 

 rectangular — it would have a diameter of 14 8 mm. If we assume 

 equal conductivities and magnetic properties, we can infer roughly 

 from the 12- and 4 -inch magnets what the effect of induced currents 

 in our specimen would be. 



Take the 12-inch magnet. For reversal of maximum H = 2*4 the 

 effects had died away in about 400 seconds. Similar events will 



/14*8\ 2 



happen in the pure iron core, but at times varying as ( -— — ) : that 



is, we should expect the effects to have subsided in 0*85 second. 



Take the 4-inch magnet. For reversal of magnetism H = 17 the 



effects had subsided in about 40 seconds: ( X ^ gi yes 0*94 

 second. 



It is, therefore, probable that the induced currents in the pure 

 Iron II may have something to do with the effects observed in this' 

 paper, although it is difficult to account for such times as 5 and 



* 'Journal of the Institution of Electrical Engineers,' vol. 24, No. 116 

 (1895). 



f 'Phil. Trans.,' A, vol. 186, pp. 93—121. 



