102 Dr. Faraday's Experimental Researches in Electricity : 



tricity has to travel in the two differing periods of time occu- 

 pied by the journeys. Hence the quickly moving core should 

 produce a far higher effect on the experimental helix than the 

 slowly moving core ; and this also I found to be the fact. 



2683. The short copper core was adjusted to the apparatus, 

 and the machine worked with its average velocity until forty 

 journeys to and from had been completed ; the galvanometer 

 needle passed 39° west. Then the machine was worked with 

 a greater rapidity, also for forty journeys, and the needle 

 passed through 80° or more west; finally, being worked at a 

 slow rate for the same number of journeys, the needle went 

 through only 21° west. The extreme velocities in this expe- 

 riment were probably as 1:6; the time in the longest case 

 was considerably less than that of one vibration of the needle 

 (2651.), so that I believe all the force in the slowest case was 

 collected. The needle is very little influenced by the swing 

 or momentum of its parts, because of the deadening effect of 

 the copper plate beneath it, and, except to return to zero, 

 moves very little after the motion of the apparatus ceases. A 

 silver core produced the same results. 



2684*. These effects of induced currents have a relation to 

 the phenomena of revulsion which I formerly described 

 (2310. 2315. 2338.), being the same in their exciting cause 

 and principles of action, and so the two sets of phenomena 

 confirm and illustrate each other. That the revulsive phe- 

 nomena are produced by induced currents, has been shown 

 before (2327. 2329. 2336. 2339.) ; the only difference is, that 

 with them the induced currents were produced by exalting 

 the force of a magnet placed at a fixed distance from the 

 affected metal; whilst in the present phasnomena, the force of 

 the magnet does not change, but its distance from the piece 

 of metal does. 



2685. So also the same circumstances which affect the 

 phenomena here affect the revulsive phenomena. A plate of 

 metal will, as a whole, be well-revulsed ; but if it be divided 

 across the course of the induced currents it is not then affected 

 (2529.). A ring helix of copper wire, if the extremities be 

 unconnected, will not exhibit the phenomena, but if they be 

 connected then it presents them (2660.). 



2686. On the whole, the revulsive phenomena are a far 

 better test and indication of these currents than the present 

 effects ; especially if advantage be taken of the division of the 

 mass into plates, so as to be analogous, or rather superior, in 

 their action to the disc cylinder cores (2659. 2661.). Plati- 

 num, palladium and lead in leaf or foil, if cut or folded into 

 squares half an inch in the side, and then packed regularly 



