supposed Polarity of Diamagnetic Bodies. 97 



up and sent on to the galvanometer is a maximum: if the 

 commutator change at the moments of maximum velocity, or 

 at any other pair of moments equidistant from the one stop or 

 the other, then the current at the commutator is a minimum, 

 or 0. 



2669. There are two or three precautions which are neces- 

 sary to the production of a pure result of this kind. In the 

 first place, the iron ought to be soft and not previously in a 

 magnetic state. In the next, an effect of the following kind 

 has to be guarded against. If the iron core be away from the 

 dominant magnet at the beginning of an experiment, then, on 

 working the machine, the galvanometer will be seen to move 

 in one direction for a few moments, and afterwards, notwith- 

 standing the continued action of the machine, will return and 

 gradually take up its place at 0°. If the iron core be at its 

 shortest distance from the dominant magnet at the beginning 

 of the experiment, then the galvanometer needle will move in 

 the contrary direction to that which it took before, but will 

 again settle at 0°. These effects are due to the circumstance, 

 that, when the iron is away from the dominant magnet, it is 

 not in so strong a magnetic state, and when at the nearest to 

 it is in a stronger state, than the mean or average state, which 

 it acquires during the continuance of an experiment ; and that 

 in rising or falling to this average state, it produces two cur- 

 rents in contrary directions, which are made manifest in the 

 experiments described. These existing only for the first 

 moments, do, in their effects at the galvanometer, then appear, 

 producing a vibration which gradually passes away. 



2670. One other precaution I ought to specify. Unless 

 the commutator changes accurately at the given points of the 

 journey, a little effect is gathered up at each change, and may 

 give a permanent deflection of the needle in one direction or 

 the other. The tongues of my commutator, being at right 

 angles to the direction of motion and somewhat flexible, 

 dragged a little in the to and from parts of the journey : in 

 doing this they approximated, though only in a small degree, 

 to that which is the best condition of the commutator for 

 gathering up (and not opposing) the currents ; and a deflec- 

 tion to the right or left appeared (2677.)* Upon discovering 

 the cause and stiffening the tongues so as to prevent their 

 flexure, the effect disappeared, and the iron v/as perfectly 

 inactive. 



2671. Such therefore are the results with an iron core, 

 and such would be the effects with a copper or bismuth core 

 if they acted by a diamagnetic polarity. Let us now con- 

 sider what the consequences would be if a copper or bismuth 



Phil, Mas. S. 3. Vol. 37. No. 2<h8. Aumist 1 850. H 



