Self-induction of Wires . 191 



lower resistance that the battery behaved as a shunted con- 

 denser, or as a shunted condenser with resistance in sequence, 

 or something similar ; and I examined the influence of the 

 frequency on the values of the effective resistance and induc- 

 tance. The change in the latter was uncertain, owing to the 

 complex balancing, but the apparent resistance was notably 

 increased by increasing the frequency, viz. from 125 to 130 

 ohms, when the frequency was raised from about 500 to about 

 800, whilst there was a small reduction in the amount of the 

 negative inductance. The effect was distinct, under various 

 changes of frequency, but was the opposite (as regards 

 resistance) of what I expected on the S.H. assumption. To 

 see whereabouts the minimum apparent resistance was (being 

 165 steady), I lowered the speed by steps. The resistance 

 went down to 113 with a slow rattle, and so there was no 

 minimum at all. The S.H. assumption had not the least 

 application to the apparent resistance, as regards the values 

 165 steady, 113 slow intermittences, although it no doubt is 

 concerned in the rise from 113 to 130 at frequency 800. 

 The balance (approximate) was some complex compromise, 

 but was principally due to a vanishing of the integral extra- 

 current. Of course in such a case as this we should employ 

 a strictly S.H. impressed force ; a remark that applies more 

 or less in all cases where the combination tested does not 

 behave as a mere coil of constant R and L. 



The other effects, due to using a battery in branch 6 as well, 

 are complex. It made little difference when the current in the 

 cell was in its natural direction ; but on reversal (by reversing 

 the battery in 6) there was a rapid fall in the resistance — for 

 instance, from 46 ohms to 18 ohms in half a minute in the 

 case of a rather used-up battery, but a comparatively small 

 fall when the battery was good. 



Besides the advantage of independence of the manner of 

 variation of the impressed force (in all cases where the re- 

 sistance and inductance do not vary with the frequency), and 

 the great ease of interpretation, the equal-ratio method gives 

 us independence of the mutual induction of 1 and 2 and of 

 3 and 4 ; and this, again, leads to another advantage of an 

 important kind. If the arrangement is at all sensitive, the 

 balance will continually vary, on account of temperature 

 inequalities occurring in experimenting, caused by the breath, 

 heat of hands, lamps, &c. Now, if the four sides of the 

 quadrilateral consist of four coils, equal in pairs, it is a 

 difficult matter to follow the temperature changes. To restore 

 a resistance-balance is easy enough ; but more than that is 

 needed, viz. the preservation of the ratio of equality. But, by 



