PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
37 
two of them were used) were excited by one cell, all the phenomena exhibited by 
figs. 40 , 41 , 42 and 43 , were distinctly exhibited. 
A considerable difference is always necessary between the strength of the current 
passing through the helix and that which excites the cores, so as to prevent the 
induction of the cores, which, of itself, would be followed by repulsion, from neutral- 
izing, or perhaps inverting, the induction of the helix. When two magnets were 
used and the helix was excited by ten cells, I found the magnetic excitement by 
one or two cells to be most advantageous ; when the cores were excited by ten, or 
even five cells, the action was always repulsive*. When four magnets were applied 
and the helix was excited by a battery of ten or fifteen cells, a power of five cells 
for the magnets was found efficient. 
The deportment of paramagnetic bodies is so well known that it might be left to 
the reader to discern that in all the cases described it is perfectly antithetical to that 
of the diamagnetic body. I have nevertheless thought it worth while to make the 
corresponding experiments with an iron bar ; to facilitate comparison the results are 
placed side by side in Plate II. with those obtained with the bar of bismuth. It 
must be left to the reader to decide whether throughout this inquiry the path of 
strict inductive reasoning has been adhered to: if this be the case, then the in- 
ference appears unavoidable, that the diamagnetic force is a polar force, the polarity of 
diamagnetic bodies being opposed to that of paramagnetic ones under the same condi- 
tions of excitement^. 
* The perfect similarity of this deportment to that of soft Iron under the same circumstances is evident. 
t I -would gladly refer to M. Plucker’s results in connexion with this subject had I been successful in ob- 
taining them ; I will here however introduce the description of his most decisive experiment in his own words. 
(See Scien. Mem. New Ser. p. 336.) 
“ From considerations of which we shall speak afterwards, it appeared to me probable that bismuth not only 
assumes polarity in the vicinity of a magnetic pole, but that it also retains the polarity for some time after the 
excitation has taken place ; or, in other words, that bismuth retains a portion of its magnetism permanently, 
as steel, unlike soft iron, retains a portion of the magnetism excited in it by induction. My conjecture has 
been corroborated by experiment. 
“ I hung a bar of bismuth, 15 millims. long and 5 millims. thick, be- 
tween the pointed poles of the large electro-magnet ; it was suspended 
horizontally from a double cocoon-thread, fig. 1 . The distance between 
the points was diminished until the bar could barely swing freely between 
them. A little rod of glass was brought near to one of the points, so that 
the bismuth bar, before the magnetism was excited, and in consequence 
of the torsion, leaned against the glass rod. On exciting the magnet by a current of three of Grove’s elements, 
the bismuth, prevented from assuming the 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. In consequence of 
this reversion, the bar of bismuth, loosening from the glass rod, moved towards the axial position, but soon turned 
and pressed against the glass as before, or in some cases having passed quite through the axial position was 
driven round with the reversed ends into the equatorial. ...This experiment, which was made with some care, 
proves that the bismuth requires time to reverse its polarity.” 
I have repeated this experiment with great care, and have obtained in part the effect described : it is perfectly 
