44 Lord Rayleigh on a permanent Deflection 



the needle produced by a single impulse was only a few degrees, 

 yet under the influence of the series of equal and opposite cur- 

 rents it remained steady at 60 or 70, and that on either side 

 of the zero-point, which had in fact become a position of un- 

 stable equilibrium. Since it took place indifferently in either 

 direction, the deflection cannot be ascribed to an inequality in 

 the alternate currents, giving on the whole a balance in one 

 direction such as, according to the experiments of Henry 

 d'Abria, might arise from imperfect contacts in the second 

 circuit. 



The first explanation which suggested itself to me was that, 

 while no doubt the currents of the two series were strictly 

 equal (numerically), the resulting impulses, or rather impul- 

 sive couples, acting on the needle might be slightly different 

 owing to the change of the latter's position in reference to the 

 coil in the small vibration which the series of currents must 

 produce, however quickly they may follow one another, which 

 would give one set an advantage over the other. Those cur- 

 rents would prevail which tend to increase the deviation of the 

 needle ; for they would have, as it were, the greatest purchase 

 on it. To make this perfectly clear, suppose either the galva- 

 nometer to be turned round, or the direction of the magnetic 

 force altered by permanent magnets, so that the position of 

 equilibrium of the needle is now no longer zero, but say 20°, 

 and then let the series of induced currents pass. There might 

 appear at first sight to be two cases, according as the first cur- 

 rent tends to diminish or increase the already existing devia- 

 tion; but the result is the same in both, and I will take for the 

 sake of illustration that in which the needle is first sent towards 

 zero. When the second instantaneous current passes, it finds 

 the needle nearer zero, and therefore acts upon it with greater 

 force than did the first ; and this process continues, so that if 

 for the moment we imagine the needle to vibrate about 20°, 

 there is an outstanding force tending to increase the deviation. 

 As this is unbalanced, the equilibrium at 20° cannot be main- 

 tained, and the needle must move further from zero: instead 

 of equilibrium, perhaps, I should say resultant equilibrium ; for 

 the rapid vibration of the needle just now referred to of course 

 goes on in any case. I worked out the mathematical theory 

 of this action fully for a tangent-galvanometer ; and for the 

 case, to which experiment is not limited, of an equal interval 

 between consecutive instantaneous currents of opposite sorts. 

 The most conspicuous result (which might, however, have been 

 anticipated) was that the effect is independent of the rapidity 

 with which the make-and-break apparatus works. As this was 

 not at all what I had inferred from the experiment, I began to 



