38 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
VI. CONCLUDING OBSERVATIONS: ON M. WEBER’S THEORY OF DIAMAGNETIC 
POLARITY*, AND ON AMP^IRE’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 phenomena of attraction and repulsion 
exhibited by magnets, and from this it was but a step to bis celebrated theory of 
molecular currents. He supposed the molecules of a magnetic body to be surrounded 
by such currents, which, however, in the natural state of the body mutually neutralized 
each other, on account of their confused grouping. The act of magnetization he 
supposed to consist in setting these molecular currents parallel to each other, and 
starting from this principle he reduced all the phenomena of magnetism to the 
mutual action of electric currents. 
If we reflect upon the experiments recorded in the foregoing pages from first to 
last ; on the inversion of magnecrystallic phenomena by the substitution of a mag- 
netic constituent for a diamagnetic ; on the analogy of the effects produced in mag- 
netic and diamagnetic bodies by compression ; on the antithesis of the rotating 
actions described near the commencement ; on the indubitable fact that diamagnetic 
bodies, like magnetic ones, owe their phenomena to an induced condition into which 
they are thrown by the influencing magnet, and the intensity of which is a function 
of the magnetic strength ; on the circumstance that this excitation, like that of soft 
iron, is of a dual character; on the numerous additional experiments which have 
been recorded, all tending to show the perfect antithesis between the two classes of 
bodies ; — we can hardly fail to be convinced that Mr. Faraday’s first hypothesis of 
diamagnetic action is the true one — that diamagnetic bodies operated on by magnetic 
forces possess a polarity “ the same in kind as, but the reverse in direction of that 
acquired by magnetic bodies.” But if this be the case, how are we to conceive of 
the physical mechanism of this polarity ? According to Coulomb’s and Poisson’s 
theory, the act of magnetization consists in the decomposition of a neutral magnetic 
fluid ; the north pole of a magnet, for example, possesses an attraction for the south 
fluid of a piece of soft iron submitted to its influence, draws the said fluid towards it, 
and with it the material particles with which the fluid is associated. To account 
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 dotted 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 per- 
fectly evident when the attention was directed towards it. On reversing the current, a little time was re- 
quired 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 dif- 
ference between M. Plucker and myself is, that I obtained the same result by simply intercepting the current as 
by reversing it. I may remark that I have submitted ordinary bismuth to the most powerful and delicate tests, 
but as yet I have never been able to detect in it a trace of that retentive power ascribed to it by M, Plucker. 
* PoGG. Ann. vol. Ixxxvii. p. 145, and Taylor’s Scien. Mem. New Ser. p. 163. 
