wider the Operation of Feeble Magnetic Forces. 231 



observations on the drift could not be obtained with this ap- 

 paratus ; but it was evident that, whilst most of the anomalous 

 action was over in 3 or 4 seconds, the final magnetic state 

 was not attained until after about 15 or 20 seconds'*. 



The operation of feebler forces was next examined, rather 

 with the expectation of finding the drift reduced in relative 

 importance. But the imposition of ■£$ H was followed by a 

 drift of 13 or 14 divisions, no very small fraction of the whole 

 action ; as was seen from the observation that the ( + ) effect 

 was now 300 divisions, of which 150 are due to the iron. 

 With 20,000 ohms in circuit, giving a force equal to ^-Jd H, 

 the drift was 6 or 7 divisions. By still further diminishing 

 the force the drift could be reduced to insignificance ; but it 

 appeared to maintain its proportion to the instantaneous effect. 

 Apart from the complication due to the drift, the magneti- 

 zation was proportional to magnetizing force from t T q H to 



Woo H or iessf. 



The question now presents itself, What is the actual value 

 of the permeability which has been proved to be a definite 

 constant for small forces ? In consequence, however, of the 

 nearness of the operative pole to the suspended needle in the 

 preceding experiments, no moderately accurate value of {a 

 can be deduced. But the observations described in Part If. 

 are sufficient to show that the constant permeability for hard 

 iron has some such value as 90 or 100, the forces then opera- 

 tive being within the prescribed limits. The fact that the 

 initial value of /-t is so large is obviously of great theoretical 

 and practical importance. Further evidence will be brought 

 forward presently in connection with observations made with 

 an arrangement better suited to an absolute determination. 



Too definite a character must not be ascribed to the above- 

 mentioned limit of £H. Below this point the deviations from 

 the law of proportionality, though mathematically existent, are 

 barely sensible. In order to understand this, it is well to con- 

 sider what happens when the limit is plainly exceeded. If a 

 force of the order H be imposed, the compensating-coil (ad- 

 justed for small forces) appears to be overpowered, and a 



* Prof. Ewing (loc. cit. § 52) describes "a time lag in magnetization," 

 especially noticeable in the softest iron and at points near the beginning 

 of the steep part of the magnetization-curv^. It should have been stated 

 that my apparatus was very hriniy supported, and, being situated under- 

 ground, was well protected from vibration. The drift or creeping did 

 not appear to be due to this cause. 



t The results here set forth were announced in a discussion following 

 Prof. Hughes's address to the Society of Telegraph Engineers on Fe- 

 bruary 11, 1886 (Journ. Tel. Eng. xv. p. 39), on the strength of prelimi- 

 nary experiments tried towards the close of 1835. 



