182 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXIII.) 
2682. When a copper, silver, or gold core is employed in place of the iron, the effect 
is very different. There is no reason to doubt, that, as regards the core itself, the 
same amount of electricity is thrown into the form of induced circulating currents 
wdthin it, by a journey to or from, whether that journey is performed quickly or 
slowly: the above experiment (2681.) in fact confirms such a conclusion. But the 
effect which is produced upon the experimental helix is not proportionate to the 
whole amount of these currents, but to the maximum intensities to which they rise. 
When the core moves slowly, this intensity is small ; when it moves rapidly, it is 
great, and necessarily so, for the same current of electricity has to travel in the two 
differing periods of time occupied 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 experiment were pro- 
bably 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 revul- 
sion 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 illus- 
trate each other. That the revulsive phenomena 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 phenomena, 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. Oil 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.). Platinum, palladium and lead in leaf 
or foil, if cut or folded into squares half an inch in the side, and then packed regularly 
