SUPPOSED POLARITY OF DIAMAGNETIC BODIES. 
173 
other times to oppose them and to neutralize their result ; and therefore it was made 
adjustible, so as to change at any period of the time or part of the motion. 
2646. With such an arrangement as this, it is known that, however powerful the 
magnet, and however delicate the other parts of the apparatus, no effect will be pro- 
duced at the galvanometer as long as the magnet does not change in force, or in its 
action upon neighbouring bodies, or in its distance from, or relation to, the experi- 
mental helix ; but the introduction of a piece of iron into the helix, or anything else 
that can influence or be influenced by the magnet, can, or ought to, show a corre- 
sponding influence upon the helix and galvanometer. My apparatus 1 should ima- 
gine, indeed, to be almost the same in principle and practice as that of M. Weber 
(2640.), except that it gives me contrary results. 
2647. But to obtain correct conclusions, it is most essential that extreme precaution 
should be taken in relation to many points which at first may seem unimportant. All 
parts of the apparatus should have perfect steadiness, and be fixed almost with the 
care due to an astronomical instrument ; for any motion of any portion of it is, from 
the construction, sure to synchronize with the motion of the commutator ; and por- 
tions of effect, inconceivably small, are then gathered up and made manifest as a 
whole at the galvanometer ; and thus, without care, errors might be taken for real 
and correct results. Therefore, in my arrangements, the machine (2643, &c.), the 
magnet and helix, and the galvanometer stood upon separate tables, and these again 
upon a stone floor laid upon the earth ; and the table carrying the machine was care- 
fully strutted to neighbouring stone-work. 
2648. Again, the apparatus should itself be perfectly firm and without shake in its 
motion, and yet easy and free. No iron should be employed in any of the moving 
parts. I have springs to receive and convert a portion of the momentum of the whole 
at the end of the to and fro journey ; but it is essential that these should be of ham- 
mered brass or copper. 
2649. It is absolutely necessary that the cylinder or core in its motion should not 
in the least degree disturb or shake the experimental helix and the magnet. Such a 
shake may easily take place and yet (without much experience) not be perceived. It 
is important to have the cores of such bodies as bismuth, phosphorus, copper, &c., as 
large as may be, but I have not found it safe to have less than one-eighth of an inch 
of space between them and the interior of the experimental helix. In order to float, 
as it were, the core in the air, it is convenient to suspend it in the bight or turn of a 
fine copper wire passing once round it, the ends of which rise up, and are made fast 
to two fixed points at equal heights but wide apart, so that the wire has a V form. 
This suspension keeps the core parallel to itself in every part of its motion. 
2650. The magnet, when excited, is urged by an electric current from five pairs of 
Grove’s plates, and is then very powerful. When the battery is not connected with 
it, it still remains a magnet of feeble power, and when thus employed may be re- 
ferred to as in the residual state. If employed in the residual state, its power may for 
