supposed Polarity of Diamagnetic Bodies. 91 



My apparatus I should imagine, 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 essen- 

 tial 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 

 portions 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 re*- 

 suits. 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 carefully 

 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 essen- 

 tial that these should be of hammered 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 experi- 

 mental 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 referred to as in the residual slate. If employed in 

 the residual state, its power may for the time be considered 

 constant, and the experimental helix may at any moment be 

 connected with the galvanometer without any current appear- 



