96 Dr. Faraday's Experimental Researches in Electricity : 



until at the greatest distance. Both this effect, and its pro~ 

 gression inwards and outwards, cause currents to be induced 

 in the surrounding helix, and these currents are in one direc- 

 tion as the core advances, and in the contrary direction as it 

 recedes. In reality, however, the iron does not travel with a 

 constant velocity ; for, because of the communication of motion 

 from a revolving crank at the machine (2643.), it, in the to 

 part of the journey, gradually rises from a state of rest to a 

 maximum velocity, which is half-way, and then as gradually 

 sinks to rest again near the magnet: — and the from part of 

 the journey undergoes the same variations. Now as the 

 maximum effect upon the surrounding experimental helix 

 depends upon the velocity conjointly with the intensity of the 

 magnetic force in the end of the core, it is evident that it will 

 not occur with the maximum velocity, which is in the middle 

 of the to ox from motion ; nor at the stop nearest to the domi- 

 nant magnet, where the core end has greatest magnetic force, 

 but somewhere between the two. Nevertheless, during the 

 "whole of the advance, the core will cause a current in the ex- 

 perimental helix in one direction, and during the whole of the 

 recession it will cause a current in the other direction. 



2666. If diamagnetic bodies, under the influence of the 

 dominant magnet, assume also a polar state, the difference 

 between them and iron being only that the poles of like names 

 or forces are changed in place (2429. 24-30.), then the same 

 kind of action as that described for iron would occur with 

 them ; the only difference being, that the two currents pro- 

 duced would be in the reverse direction to those produced by 

 iron. 



2667. If a commutator, therefore, were to be arranged to 

 gather up these currents, either in the one case or the other, 

 and send them on to the galvanometer in one consistent cur- 

 rent, it should change at the moments of the two stops (2665.), 

 and then would perform such duty perfectly. If, on the other 

 hand, the commutator should change at the times of maximum 

 velocity or maximum intensity, or at two other times equi- 

 distant either from the one stop or from the other, then the 

 parts of the opposite currents intercepted between the changes 

 would exactly neutralize each other, and no final current 

 would be sent on to the galvanometer. 



2668. Now the action of the iron is, by experiment, of this 

 nature. If an iron wire be simply introduced or taken out of 

 the experimental helix with different conditions of the com- 

 mutator, the results are exactly those which have been stated. 

 If the machine be worked with an iron wire core, the commu- 

 tator changing at the stops (2665.), then the current gathered 



