30 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
Here is a source of error, against which, however, M. v. Feilitsch has probably 
guarded himself. Some irregularity of crystalline structure may, perhaps, have in- 
fluenced the result. With “ chemically pure zinc” M. v. Feilitsch obtained the same 
deflection that I obtained with bismuth ; now chemically pure zinc is diamagnetic*, 
and hence its deportment is corroborative of that which I have observed. M.v. Fei- 
litsch, however, appears to regard the zinc used by him as magnetic ; but if this be 
the case it cannot have been chemically pure. It is necessary to remark that I have 
called the north pole of the electro-magnet that which attracts the south, or unmarked 
end, of a magnetic needle ; and I believe this is the custom throughout Germany. 
Abnormal Paramagnetic Bar. — This bar consisted of compressed carbonate of iron 
dust, and was suspended within the helix with the line of compression, which was its 
shortest dimension, horizontal. As in the cases already described, it was first acted 
upon by the magnet alone ; having attained its position of equilibrium, a current was 
sent through the helix, and the subsequent deflection was observed. 
The magnet being excited as in fig. 17, Plate I., the bar set its length equatorial ; 
on sending a current through the helix in the direction of the arrow, the bar was 
deflected to the dotted position. 
Reversing the current in the helix, but permitting the magnet to remain as before, 
the deflection was that shown in fig. 18. 
Interrupting all, and reversing the polarity of the magnet ; on sending the current 
through as in the last case, the deflection was that shown in fig. 19. 
Reversing the current, but preserving the last condition of the magnet, the deflec- 
tion was that shown in fig. 20. 
In the subsequent four experiments the helix was excited first. Whatever might 
be the direction of the current through the helix, the bar always set its length perpen- 
dicular to the axis of the latter, and parallel to the coils. 
When the direction of the helix current, and the polarity of the magnet, were those 
shown in fig. 21, the deflection was to the dotted position. 
Interrupting all, and reversing the current in the helix ; on exciting the magnet the 
deflection was that shown in fig. 22. 
Changing the polarity of the magnet, and preserving the helix current in its former 
direction, the deflection was that shown in fig. 23. 
Interrupting all, and reversing the current through the helix ; when the magnetism 
was developed the deflection was that shown in fig. 24. 
Abnormal Diamagnetic Bar . — This bar consisted of a prism of bismuth whose prin- 
cipal planes of crystallization were perpendicular to its length : the mode of experi- 
ment was the same as that applied in the other cases. 
Acted upon by the magnet alone, the bar set its length from pole to pole : the mag- 
netic excitation being that denoted by fig. 29, a current was sent through the helix in 
the direction of the arrow ; the bar was deflected to the dotted position. 
* Phil. Mag. vol. xxviii. p. 456. 
