705 



importance. The double deviation remained unchanged when the 

 position of the axis of the coil with respect to a vertical line was 

 changed by means of the foot screws, a change which gave rise to 

 horizontal alternating fields. 



Effect III, however, which was caused by the action which stati- 

 onary magnetic fields can exert on the alternating poles on account 

 of their excenfric position could easily be observed. The double 

 deviation changed immediately when a permanent magnet was 

 brought near the coil. The influence of the terrestrial magnetism 

 was also apparent. When it was not compensated we got, in the 

 case of resonance, a broadening of the image on the scale up to 

 3 em for a scale distance of 45 cm. In all further experiments 

 the terrestrial field has therefore been compensated, the measure- 

 ments required for this being made with an earth inductor and a 

 ballistic galvanometer. The horizontal and vertical components of 

 the terrestrial field were compensated separately by means of hoops 

 of about 1 m. diameter on which copper wire was wound. The 

 current was taken from storage cells, and precision Ampèremeters of 

 Siemens and Halske served for continually controlling its strength. 



Whether the compensation was obtained could be tested l)y turning 

 the upper end of the suspending wire. The amplitude of the oscil- 

 lations changed by this so long as the terrestrial magnetism was 

 still acting on the iron magnetized by the alternating current. After 

 compensation however this azimuthal sensibility of the elfect had 

 disappeared. After all there remained a well marked double devi- 

 ation of 4,5 mm. 



We now had to make sure that this was really the effect we 

 sought for. For this purpose we first availed ourselves of the cir- 

 cumstance that the acting couple must differ a quarter of a period 

 in phase from the current and the magnetisation. We brought a 

 permanent magnet near the coil, thereby calling forth effect III 

 and adding to the couple B^ cos lot, with which we are conceriied, 

 a new one, which has the same or the opposite phase as (he magne- 

 tisation and therefore diflers a quarter of a period in phase from 

 B^ cos lot. Whatever be the sign of this additional couple, the ampli- 

 tude of the resulting one must become larger than B^. We found 

 indeed that the broadening of the image always increased when we 

 brought a magnet near the coil. 



Further the theory requires that the magnitude of the effect depends 

 on the intensity of the alternating field in the same way as the 

 magnetisation itself. This was likewise confirmed by experiment. 



Finally we shall compare the observed magnitude of the effect 



