372 



Mr. E. Wilson. 



the opposite maximum, the sum does not equal tbe whole kick which 

 would be observed if the force were at once varied from the one 

 maximum to the other. Daring the interval the magnetism appears 

 to continue to settle down, so that the change which lastly takes 

 place is not so great as it would be if such apparent settling down 

 did not occur. 



Experiments were made to investigate the effect when the limits 

 of B were (a) large and (b) small. It is assumed that the instru- 

 ment gives the true time integral of current. 



(a) Maximum B = 15,270, coercive force 1*13 C.G.S. units. The 

 maximum force H of 9*24 C.G.S. units was suddenly varied through 

 zero to 1*13, and the secondary circuit kept closed until deflections 

 to the left and right were observed, the periodic time of the galvano- 

 meter needle being 10"6 seconds. The scale is graduated from on 

 the left to 1000 on the right, and the readings taken were 351, 623, 

 giving a difference of 272, corresponding to a change of induction 

 per square centimetre of 12,630 C.G.S. units. When the magnetism 

 had settled down, as was shown by closing the secondary key with no 

 extra resistance in its circuit, and observing no deflection on the 

 ballistic galvanometer, a suitable extra resistance was inserted, and 

 the force suddenly raised to its maximum value, the observed deflec- 

 tions were 362, 627, the difference 265 corresponding to B = 12,350. 

 These results were many times repeated. 



The total change of induction produced a deflection 662, 330, the 

 difference 332 corresponding to B = 15,270. We have therefore to 

 account for a difference of 5560, or 18 per cent, of the total change 

 from one maximum to the ether. The zero, when the spot of light 

 is perfectly steady, is 495, and we can see that when making the 

 first change from one maximum through zero to force 1 13 the 

 deflection to the left is 143 as against 128 to the right; whereas 

 when making the second change the deflections are 133 to the left 

 and 132 to right. There is evidence here of a change continuing in 

 the same direction, since the first elongation is greater than the 

 second, and the decrement would only account for about 1 per cent. 



This effect was next observed in a slightly different manner. The 

 change of force from one maximum through zero to the then 

 coercive force was effected, and the secondary circuit closed at 

 known intervals of time after such change. The results are given 

 in Table III. 



It will be seen from the figures that about 30 per cent, comes out 

 after the first second has elapsed, and that the result is practically 

 the same, whether the charging potential difference be that due to 

 ten or fifty- six cells. With a total reversal from one maximum to 

 the other no such effect was observed, the change taking place 

 immediately. Having taken the force from one maximum through 



