body, may be increased and diminished; and is sometimes far stronger, for the quantity of matter, than the power of gravity; and in receding from the magnet, decreases not in the duplicate, but almost in the triplicate proportion of the distance, as nearly as I could judge from some rude observations,"
The edition of the Principia which was published in 1742 by Thomas Le Seur and Francis Jacquier contains a note on this corollary, in which the correct result is obtained that the directive couple exercised on one magnet by another is proportional to the inverse cube of the distance.
The first discoverer of the law of force between magnetic t poles was John Michell (b. 1724, d. 1793), at that time a young Fellow of Queen's College, Cambridge,[1] who in 1750 published A Treatise of Artificial Magnets; in which is shown an easy and expeditious method of making them superior to the best natural ones. In this he states the principles of magnetic theory as follows[2]:—
"Wherever any Magnetism is found, whether in the Magnet itself, or any piece of Iron, etc., excited by the Magnet, there are always found two Poles, which are generally called North and South; and the North Pole of one Magnet always attracts the South Pole, and repels the North Pole of another; and vice versa."
This is of course adopted from Gilbert.
"Each Pole attracts or repels exactly equally, at equal distances, in every direction." This, it may be observed, overthrows the theory of vortices, with which it is irreconcilable. "The Magnetical Attraction and Repulsion are exactly equal to each other." This, obvious though it may seem to us, was really a most important advance, for, as he remarks, "Most people, who
- ↑ Michell had taken bis degree only two years previously. Later in life he was on terms of friendship with Priestley, Cavendish, and William Herschel; it was he who taught Herschel the art of grinding mirrors for telescopes. The plan of determining the density of the earth, which was carried out by Cavendish in 1999, and is generally known as the "Cavendish Experiment," was due to Michell. Michell was the first inventor of the torsion-balance; he also made many valuable contributions to Astronomy. In 1767 he became Rector of Thornhill, Yorks, and lived there until his death.
- ↑ 4 Loc. cit., p. 17.
