252.  When the marked pole was put at the side of the iron plate at a certain distance, the number of vibrations was only five.  When the marked pole of the second bar was put on the opposite side of the plate at the same distance (250.), the vibrations were reduced to two.  But when the second pole was an unmarked one, yet occupying exactly the same position, the vibrations rose to twenty-two.  By removing the stronger of these two opposite poles a little way from the plate, the vibrations increased to thirty-one, or nearly the original number.  But on removing it altogether, they fell to between five and six.

253.  Nothing can be more clear, therefore, than that with iron, and bodies admitting of ordinary magnetic induction, opposite poles on opposite sides of the edge of the plate neutralize each other’s effect, whilst similar poles exalt the action; a single pole end on is also sufficient.  But with copper, and substances not sensible to ordinary magnetic impressions, similar poles on opposite sides of the plate neutralize each other; opposite poles exalt the action; and a single pole at the edge or end on does nothing.

254.  Nothing can more completely show the thorough independence of the effects obtained with the metals by Arago, and those due to ordinary magnetic forces; and henceforth, therefore, the application of two poles to various moving substances will, if they appear at all magnetically affected, afford a proof of the nature of that affection.  If opposite poles produce a greater effect than one pole, the result will be due to electric currents.  If similar poles produce more effect than one, then the power is not electrical; it is not like that active in the metals and carbon when they are moving, and in most cases will probably be found to be not even magnetical, but the result of irregular causes not anticipated and consequently not guarded against.

255.  The result of these investigations tends to show that there are really but very few bodies that are magnetic in the manner of iron.  I have often sought for indications of this power in the common metals and other substances; and once in illustration of Arago’s objection (82.), and in hopes of ascertaining the existence of currents in metals by the momentary approach of a magnet, suspended a disc of copper by a single fibre of silk in an excellent vacuum, and approximated powerful magnets on the outside of the jar, making them approach and recede in unison with a pendulum that vibrated as the disc would do:  but no motion could be obtained; not merely, no indication of ordinary magnetic powers, but none or any electric current occasioned in the metal by the approximation and recession of the magnet.  I therefore venture to arrange substances in three classes as regards their relation to magnets; first, those which are affected when at rest, like iron, nickel, &c., being such as possess ordinary magnetic properties; then, those which are affected when in motion, being conductors of electricity in which are produced electric currents by the inductive force of the magnet; and, lastly, those which are perfectly indifferent to the magnet, whether at rest or in motion.