38 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XX.) 
therefore, as there is no part by which the return of the current can be carried on, 
no current can be formed. 
2332. Before proceeding to the explanation of the other phenomena, it will be 
necessary to point out the fact generally understood and acknowledged, I believe, 
that time is required for the development of magnetism in an iron core by a current 
of electricity; and also for its fall back again when the current is stopped. One 
effect of the gradual rise in power was referred to in the last series of these Researches 
( 2170 .). This time is probably longer with iron not well annealed than with very 
good and perfectly annealed iron. The last portions of magnetism which a given 
current can develope in a certain core of iron, are also apparently acquired more 
slowly than the first portions ; and these portions (or the condition of iron to which 
they are due) also appear to be lost more slowly than the other portions of the power. 
If electric contact be made for an instant only, the magnetism developed by the 
current disappears as instantly on the breaking of the current, as it appeared on its 
formation ; but if contact be continued for three or four seconds, breaking the contact 
is by no means accompanied by a disappearance of the magnetism with equal rapidity. 
2333. In order to trace the peculiar effect of the copper, and its cause, let us con- 
sider the condition of the horizontal bar (2310. 2313.) when in the equatorial position, 
between the two magnetic poles, or before a single pole ; the point of suspension 
being in a line with the axis of the pole and its exciting wire helix. On sending an 
electric current through the helix, both it and the magnet it produces will conduce 
to the formation of currents in the copper bar in the contrary direction. This is 
shown from my former researches (26.), and may be proved, by placing a small or 
large wire helix-shaped (if it be desired) in the form of the bar, and carrying away 
the currents produced in it, by wires to a galvanometer at a distance. Such currents 
being produced in the copper, only continue whilst the magnetism of the core is 
rising and then cease (18.39.), but whilst they continue, they give a virtual magnetic 
polarity to that face of the copper bar which is opposite to a certain pole, the polarity 
being the same in kind as the pole it faces. Thus on the side of the bar facing the 
north pole of the magnet, a north polarity will be developed ; and on that side 
facing the south pole, a south polarity will be generated. 
2334. It is easy to see that if the copper during this time were opposite only one 
pole, or being between two poles, were nearer to one than the other, this effect would 
cause its repulsion. Still, it cannot account for the whole amount of the repulsion 
observed alike with copper as with bismuth (2295.), because the currents are of but 
momentary duration, and the repulsion due to them would cease with them. They 
do, however, cause a brief repulsive effort, to which is chiefly due the first part of the 
peculiar effect. 
2335. For if the copper bar, instead of being parallel to the face of the magnetic 
pole, and therefore at right angles to the resultant of magnetic force, be inclined, 
