PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
47 
acting upon the former be <p, and the attractive force acting upon the latter (p\ It is 
manifest that if were equal to (p', as in the case of the earth’s action, or in other 
words, if the field of force were perfectly uniform, then, owing to the greater distance 
of cp' from the axis of rotation, from the moment at which the rectangular box quits 
the equatorial position, which is one of unstable equilibrium, to the moment when 
its position is axial, the box would be incessantly drawn towards the position last 
referred to. 
But it will be retorted that the field of force is not uniform, and that the end h, on 
account of its greater proximity to the magnet, is more forcibly repelled than the 
end f is attracted : to this I would reply, that it is only in “ fields” which are 
approximately uniform that the effects can be produced ; but to produce motion 
towards the pole, it is not necessary that the field should be perfectly uniform : setting, 
as before, the distance of the direction of the force p from the axis of rotation =d, and 
that of the force p’—d', a motion towards the pole N will always occur whenever 
d’ 
To ascertain the diminution of the force on receding from a polar surface such as 
that here used, I suspended a prism of bismuth, similar to those contained in the 
rectangular box, at a distance of 0*9 of an inch from the surface of the pole. Here, 
under the action of the magnet excited by a current of ten cells, the number of oscil- 
lations accomplished in a second was 17 ; at 0*7 of an inch distant the number 
was 18; at 0*5 of an inch distant the number i^as 19; at 0*3 distant the number 
was 19*5 ; and at 0*2 distant the number was 20. The forces at these respective 
distances being so very little different from each other, it follows that a very slight 
deviation of the box from the equatorial position is sufficient to give the moment of 
p' a preponderance over that of p, and consequently to produce the exact effect 
observed in the experiment. 
The consistency of this reasoning is still further shown when we operate in a field 
of force which diminishes speedily in intensity as we recede from the magnet. 
Such a field is the space immediately in front of pointed poles. Suspending our 
rectangular box between the points, and causing the latter to approach until the box 
has barely room to swing between them, it is impossible to produce the phenomena 
which we have just described. The intensity with which the nearest points of the 
bismuth bar are repelled so much exceeds the attraction of the more distant end, 
that the moment of attraction is not able to cope successfully with the moment of 
repulsion ; the bars are consequently repelled en masse, and the length of the box 
takes up a position at right angles to the line which unites the poles. 
It is manifest, however, that by increasing the distance between the bismuth bar 
and the points acting upon it, we diminish the difference of action upon the two ends 
of the bar. When the distance is sufficient, we can produce, with the pointed poles, 
all the phenomena exhibited between fiat or rounded ones. 
