Michael Faraday, His Life and Work

by Silvanus P. Thompson

“A piece of heavy glass, which was 2 inches by 1·8 inches and 0·5 of an inch thick, being a silico-borate of lead, was experimented with. It gave no effects when the same magnetic poles or the contrary poles were on opposite sides (as respects the course of the polarised ray);—nor when the same poles were on the same side either with the constant or intermitting current; BUT when contrary magnetic poles were on the same side there was an effect produced on the polarised ray, and thus magnetic force and light were proved to have relations to each other. This fact will most likely prove exceedingly fertile, and of great value in the investigation of conditions of natural force.

“The effect was of this kind. The glass, a result of one of my old experiments on optical glass, had been exceedingly well annealed so that it did not in any degree affect the polarized ray. The two magnetic poles were in a horizontal plane, and the piece of glass put up flat against them so that the polarized ray could pass through its edges and be examined by the eye at a Nicholl’s eye piece. In its natural state the glass had no effect on the polarized ray but on making contact at the battery so as to render the cores N and S magnets instantly the glass acquired a certain degree of power of depolarizing the ray which it retained steadily as long as the cores were magnets but which it lost the instant the electric current was stopped. Hence it was a permanent condition and as was expected did not sensibly appear with an intermitting current.

Fig. 17.

“The effect was not influenced by any jogging motion or any moderate pressure of the hands on the glass.

“The heavy glass had tinfoil coatings on its two sides but when these were taken off the effect remained exactly the same.

“A mass of soft iron on the outside of the heavy glass greatly diminished the effect [see [Fig. 17]]....

“All this shews that it is when the polarized ray passes parallel to the lines of magnetic induction or rather to the direction of the magnetic curves, that the glass manifests its power of affecting the ray. So that the heavy glass in its magnetized state corresponds to the cube of rock crystal: the direction of the magnetic curves in the piece of glass corresponding to the direction of the optic axis in the crystal (see Exp. Researches 1689–1698)....

Fig. 18.