PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
33 
produce the effects observed. From this point of view all the movements of diamag- 
netic bodies become perfectly intelligible, and the experiments to be recorded in the 
next chapter are not calculated to diminish the probability of the conclusion that 
diamagnetic bodies possess a polarity opposed to that of magnetic ones. 
The phenomena to which we have thus far referred consist in the rotations of elon- 
gated bars about their axes of suspension. The same antithesis, however, presents 
itself when we compare the motion of translation of a paramagnetic body, within the 
coil, with that of a diamagnetic one. A paramagnetic sphere was attached to the 
end of a horizontal beam and introduced into the coil : the magnet being excited the 
sphere could be made to traverse the space within the coil in various directions, by 
properly varying the current through the coil. A diamagnetic sphere was submitted 
to the same examination, and it was found that the motions of both spheres, when 
operated on by the same forces, were always in opposite directions. 
V. FURTHER COMPARISON OF PARAMAGNETIC AND DIAMAGNETIC PHENOMENA:— 
DIAMAGNETIC POLARITY. 
When an iron bar is placed within a helix, it is well known that on sending a current 
through the latter the bar is converted into a magnet, one end of the bar thus excited 
being attracted, and the other end repelled by the same magnetic pole. In this two- 
ness of action consists what is called the polarity of the bar : we will now consider 
whether a bar of bismuth exhibits similar effects. 
Fig. 39 Plate II. represents the disposition of the apparatus used in the examination 
of this question. AB is a helix of covered copper wire one-fifteenth of an inch in 
thickness : the length of the helix is 5 inches, external diameter 5 inches, and internal 
diameter 1*5 inch. Within this helix a bar of bismuth 6^ inches long and 0‘4 of an inch 
thick was suspended. The suspension was effected by. means of a light beam, from two 
points of which, sufficiently distant from each other, depended two silver wires each 
ending in a loop : into these loops, //', the bar of bismuth was introduced, and the whole 
was suspended by a number of fibres of unspun silk from a suitable point of support. 
Fig. 39 a is a side view of the arrangement used for the suspension of the bar. Before 
introducing the latter within the helix, it was first suspended in a reeeiver, which 
protected it from air currents, and in which it remained until the torsion of the fibre 
had exhausted itself: the bar was then removed, and the beam, without permitting 
the fibre to twist again, was placed over the helix so as to receive the bar introduced 
through the latter. From the ends of this helix two wires passed to a current 
reverser R, from which they proceeded further to the poles of a voltaic battery. 
CD and EF are two electro-magnetic spirals, each 12 inches long, 5 ^ inches external 
and 2 inches internal diameter. The wire composing them is one-tenth of an inch 
thick, and so coiled that the current could be sent through four wires simulta- 
neously. Within these spirals were introduced two cores of soft iron 2 inches thick 
and 14 inches long: the ends of the cores appear at P and P'. The spirals were so 
MDCCCLV. F 
