274 Prof. Tyndall on the Nature of the Force by which 



VI. Concluding Observations : on M. Weber's Theory of 



DlAMAGNETIC PoLARITY*, AND ON AmPERe's ThEORY OF 



Molecular Currents. 



It is well known that a voltaic current exerts an attractive 

 force upon a second current, flowing in the same direction ; and 

 that when the directions are opposed to each other the force 

 exerted is a repulsive one. By coiling wires into spirals, Ampere 

 was enabled to make them produce all the phfenomena of attrac- 

 tion and repulsion exhibited by magnets, and from this it was 

 bvit a step to his celebrated theoiy of molecular currents. He 

 supposed the molecules of a magnetic body to be surrounded by 

 such cun-ents, which, however, in the natural state of the body 



Fig. 1. 



tally from a double cocoon-thread, fig. 1 . The distance between the points 

 was diminished until the bar 

 could barely swing freely be- 

 tween them. A little rod of 

 glass was brought near to one 

 of the points, so that the bis- 

 muth bar,before the magnetism 

 was excited, and in consequence 

 of the torsion, leaned against the glass rod. On exciting the magnet by a 

 eiu'rent of three of Grove's elements, the bismuth, prevented from assuming 

 (he equatorial position, pressed more forcibly against the glass rod; when 

 the current was interrupted, the bar remained still in contact with the rod, 

 while its free end vibrated round its position of equilibrium. The current 

 was closed anew and then reversed by a gyrotrope. Inconsequence of this 

 reversion, the bar of bismuth, loosening from the glass rod, moved towards 

 the axial position, but soon tin-ned and pressed against the glass as before, 

 or in some cases having passed quite through the axial position was driven 



round mth the reversed ends into the equatorial This experiment, 



which was made with some care, proves that the bismuth requires time to 

 reverse its polarity." 



1 have repeated this experiment with great care, and have obtained in 

 part the effect described : it is perfectly easy to produce the rotation of the 

 bar. The cause of this rotation, however, was in my case as follows : — 

 When the magnet was unexcited, the position of equilibrium of the axis of 

 the bar acted upon by the torsion of the fibre was that shown by the dot- 

 ted line in the figure ; when the magnetism was developed, the repulsive 

 force acting on the free end of the bar necessarily pushed it beyond the 

 dotted line— an action which was perfectly evident when the attention was 

 directed towards it. On reversing the current, a little time was requii'cd 

 to change the polarity of the iron masses; during this time the free end of 

 the bismuth fell towards its former position, and the velocity acquired was 

 sufficient to carry it quite beyond the pole points. The only difference 

 between M. Pliicker and myself is, that I obtained the same result by 

 simply intercepting the current as by reversinf^ it. I may remark- that I 

 have submitted ordinary bismuth to the most powerful and dehcate tests, 

 but as yet 1 have never been able to detect in it a trace of that retentive 

 ))owcr ascribed to it by M. Pliicker. 



* Poggendorff's AnnaUn, vol. Ixxxvii. p. 145, and Taylor's Scientific 

 Memoirs, New Ser. p. 163. 



