oj the Galvanometer-needle. 45 



doubt whether I had hit upon the true cause of the pheno- 

 menon ; and on more close examination of the mathematical 

 result, it appeared that the needle could not remain perma- 

 nently deflected from its position of equilibrium at zero, unless 

 each instantaneous current was powerful enough to swing it 

 right round when acting on it alone, although an already ex- 

 isting deviation would be always increased. I have already 

 mentioned that the phenomenon was observed when the swing 

 for a single current was only a few degrees, so that there is no 

 doubt of the inadequacy of the foregoing explanation. 



The real cause is, I believe, to be found in a deficiency in the 

 hardness of the steel needles, rendering them to some extent 

 capable of temporary magnetism when placed in a field of 

 force. If this temporary magnetism alone be considered, the 

 two sets of instantaneous currents conspire in their effects 

 instead of opposing each other ; for if a soft-iron needle be 

 freely suspended in a uniform field of magnetic force, it has, 

 as is known, four positions of equilibrium, of which those two 

 are stable which would be positions of equilibrium (one stable 

 and one unstable) for a magnetized steel bar. If while the 

 needle is in equilibrium the direction of the magnetic force is 

 reversed, no disturbance takes place, because the magnetism of 

 the needle is at the same time reversed also. If such a needle 

 be suspended in the coil of a galvanometer, the force with 

 which a current acts upon it is independent of the direction 

 and varies as the square of the current ; or when there is a 

 rapid series of varying but periodic currents, the deflecting 

 force varies as the integral of the square of the current, and 

 as the sine of twice the deviation from zero. The deflecting 

 force would, according to this, be for a given position of the 

 needle with reference to the coil (or deviation) proportional to 

 the heating-power of the discontinuous current ; but it must 

 be remembered that the case is an ideal one, as no iron is per- 

 fectly soft or capable of at once assuming the magnetism due 

 to the field of force in which it is placed. A remarkable illus- 

 tration of this will be mentioned a little later. 



In order to test the correctness of these views, I removed 

 the steel needles from the galvanometer and replaced them by 

 a single soft-iron needle, with which it was found that all the 

 phenomena observed before were reproduced. Being anxious 

 to submit the arrangement to a more severe test, I placed the 

 galvanometer in a third circuit, so that it should be acted on 

 by the currents induced by the induced currents of the second 

 circuit, as in Henry's experiments. The effect was very marked, 

 though for this it was necessary that the galvanometer should 

 be turned round so that the position of equilibrium should be 



