26 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
When a fine fibre and sufficient power are made use of, this deportment is obtained 
without difficulty. 
An abnormal diamagnetic bar was suspended as above ; on the passag^e of the cur- 
rent it acted precisely as the normal magnetic bar ; it set its length parallel to the 
axis of the helix and perpendicular to the coils. Here also, by fine manipulation, the 
result is obtained with ease and certainty. 
Action of Magnet and Current combined. — In examining this subject eight experi- 
ments were made with each particular bar ; it will be remembered that fig. 6 gives a 
general view of the arrangement. 
1. Four experiments were made in which the magnet was excited first, and 
after the suspended bar had taken up its position of equilibrium, the deflection 
produced by the passage of a current through the surrounding helix was ob- 
served. 
2. Four experiments were made in which the helix was excited first, and when 
the bar within it had taken up its position of equilibrium, the magnetism was deve- 
loped and the consequent deflection observed. 
Normal Paramagnetic Bar. — In experimenting with the soft iron it was necessary 
to place it at some distance from the magnet ; 
otherwise the attraction of the entire mass by one 
or the other pole would completely mask the ac- 
tion sought. Fig. 7 represents the disposition of 
things in these experiments : N and S indicate the 
north and south poles of the magnet ; ab is the 
bar of iron ; the helix within which the bar was 
suspended is shown in outline around it ; the 
arrow shows the direction of the current in the 
upper half oi the helix; its direction in the under 
portion would, of course, be the reverse. 
On exciting the magnet, the bar of soft iron set itself parallel to the line join- 
ing the poles, as shown by the unbroken line in fig. 7- 
When the direction of the current in the helix Fig- 8- 
was that indicated by the arrow, the bar was de- 
flected towards the position dotted in the figure. 
Interrupting the current in the helix, and per- 
mitting the magnet to remain excited, the bar 
returned to its former position : the current was 
now sent through the helix in the direction of 
the arrow, fig. 8 ; the consequent deflection was 
towards the dotted position. 
Both the current which excited the magnet and that which passed through the 
Fig. 7. 
