242 
PROFESSOR TYNDALL’S FURTHER RESEARCHES 
same pole of a magnetic needle ; and that between both there is a neutral point which 
neither attracts nor repels. The same is the case with the helices HE, H'E'; so that 
when a current is sent through them, if the astatic magnet be exactly opposite the 
neutral point, it is unaffected by the helices. This is scarcely attainable in practice ; 
a slight residual action remains which draws the magnets against the helices ; but 
this is very easily neutralized by disposing an external portion of the circuit so as to 
act upon the magnets in a direction opposed to that of the residual action. Here 
then we have a pair of spirals which, when excited, do not act upon the magnets, and 
which therefore permit us to examine the pure action of any body capable of magnetic 
excitement placed within them. In the experiments to be described, it was always 
arranged that the current flowed in opposite directions through the two spirals; so 
that if the bodies within them were polar, the two upper ends of these bodies should 
be poles of opposite names, and consequently the two lower ends opposed also. Sup- 
posing now our two cylinders to occupy the central position indicated in fig. 2 : even 
if the cylinders became polar through the action of the surrounding current, the 
magnets, being opposite to the neutral points of the cylinders, would experience no 
action from the latter. But suppose the wheel W' to be so turned that the two cylin- 
ders are brought into the position shown in fig. 1, the upper end 
o of op and the lower end n of mn will act simultaneously upon 
the suspended magnets. For the sake of illustration, let us sup- 
pose the ends o and n to be both north poles, and that the sec- 
tion, fig. 4, is taken when the bars are in the position shown in 
fig. 1. The right-hand pole o will attract S' and repel N, which 
attraction and repulsion sum themselves together to produce a deflection of the system 
of magnets. On the other hand, the left-hand pole n, being also north, will attract 
S and repel N', which two effects also sum themselves to produce a deflection in the 
same direction as the former two. Hence, not only is the action of terrestrial mag- 
netism annulled by this arrangement, but the moving force due to the reciprocal 
action of the magnets and the bodies within the helices is increased fourfold. By turn- 
ing the wheel in the other direction we bring the cylinders into the position shown in 
fig. 3, and thus may study the action of the ends m and p upon the magnets. 
By means of the screw R the magnets can be raised or lowered ; and at the end, t, 
of the screw is a small torsion circle which can be turned independently of the screw; 
by means of the latter the suspending fibre can be twisted or untwisted without 
altering the level of the magnets. 
The front of the box is attached by means of brass hasps, and opposite to the 
mirror M a small plate of glass is introduced, through which the mirror is observed ; 
the magnets within the box being thus effectually protected from the disturbances 
of the external air. A small handle to turn the wheel W' accompanied the instru- 
ment from its maker; but in the experiments, I used, instead of it, a key attached 
to the end of a rod 10 feet long; with this rod in my right hand, and the telescope 
Fig. 4. 
W 
m; 
