Michael Faraday, His Life and Work

by Silvanus P. Thompson

The discovery of diamagnetism which Faraday thus announced was in itself a notable achievement. As Tyndall points out, the discovery itself was in all probability due to Faraday’s habit of not regarding as final any negative result of an experiment until he had brought to bear upon it the most powerful resources at his command. He had tried the effects of ordinary magnets on brass and copper and other materials commonly considered as non-magnetic. But when, for the purpose of the preceding research on the relation of magnetism to light, he had deliberately procured electromagnets of unusual power, he again tried what their effect might be upon non-magnetic stuffs. Suspending a piece of his heavy glass near the poles in a stirrup of writing-paper slung upon the end of a long thread of cocoon silk, he found it to experience a strong mechanical action when the magnet was stimulated by turning on the current. His precision of description is characteristic:—

I shall have such frequent occasion to refer to two chief positions of position across the magnetic field, that, to avoid periphrasis, I will here ask leave to use a term or two conditionally. One of these directions is that from pole to pole, or along the lines of magnetic force, I will call it the axial direction; the other is the direction perpendicular to this, and across the line of magnetic force and for the time, and as respects the space between the poles, I will call it the equatorial direction.

Note the occurrence in the above passage for the first time of the term “the magnetic field.” Faraday’s description of the discovery continues as follows:—

The bar of silicated borate of lead or heavy glass already described as the substance in which magnetic forces were first made effectually to bear on a ray of light, and which is 2 inches long, and about 0·5 inch wide and thick, was suspended centrally between the magnetic poles, and left until the effect of torsion was over. The magnet was then thrown into action by making contact at the voltaic battery. Immediately the bar moved, turning round its point of suspension, into a position across the magnetic curve or line of force, and, after a few vibrations, took up its place of rest there. On being displaced by hand from this position it returned to it, and this occurred many times in succession.

Fig. 19.

Either end of the bar indifferently went to either side of the axial line. The determining circumstance was simply inclination of the bar one way or the other to the axial line at the beginning of the experiment. If a particular or marked end of the bar were on one side of the magnetic or axial line when the magnet was rendered active, that end went further outwards until the bar had taken up the equatorial position....

Here, then, we have a magnetic bar which points east and west in relation to north and south poles—i.e. points perpendicularly to the lines of magnetic force....

DIAMAGNETIC LAWS.

To produce these effects of pointing across the magnetic curves, the form of the heavy glass must be long. A cube or a fragment approaching roundness in form will not point, but a long piece will. Two or three rounded pieces or cubes, placed side by side in a paper tray, so as to form an oblong accumulation, will also point.