“Superficially at least the forces of electricity and magnetism seem to present the same kind of problem as gravitation. Experiment shows that two electrically charged bodies attract one another (or repel if their charges are of the same kind) with a force which conforms to the same mathematical law as the force of gravitation - both forces fall off inversely as the inverse square of the distance. The same is true of the magnetic force also; two magnetic poles attract or repel one another with a force which again follows the law of the inverse square of the distance.”

— James Jeans

Physics and Philosophy (1942)

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James Jeans 54

British mathematician and astronomer 1877–1946

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“Gravitational force is simple, and a thing by itself, as also are electric and magnetic forces as long as the electric and magnetic poles stand at rest. But as soon as motion comes into the picture, the whole situation is changed. Forces of new kinds come into play, for moving electric charges exert magnetic forces in addition to the electric forces they exert when at rest, while moving magnets exert electric forces in addition to the magnetic forces they exert while at rest. When the exact laws governing these intricate laws had been discovered by a great number of experimenters, Clerk Maxwell succeeded in expressing them in a mathematical form which was both simple and elegant.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“In electromagnetism… the law of the inverse square had been supreme, but, as a consequence of the work of Faraday and Maxwell, it was superseded by the field. And the same change took place in the theory of gravitation. By and by the material particles, electrically charged bodies, and magnets which are the things that we actually observe come to be looked upon only as "singularities" in the field. So far this transformation from the force to the potential, from the action at a distance to the field, is only a purely mathematical operation.”

Willem de Sitter (1872–1934) Dutch cosmologist

Kosmos (1932), Above is Beginning Quote of the Last Chapter: Relativity and Modern Theories of the Universe -->

“As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter.”

J. J. Thomson (1856–1940) British physicist

"Cathode rays" http://web.lemoyne.edu/~GIUNTA/thomson1897.html Philosophical Magazine, 44, 293 (1897).

“Electric and magnetic forces. May they live for ever, and never be forgot”

Oliver Heaviside (1850–1925) electrical engineer, mathematician and physicist

Electromagnetic Theory (1912), Volume III; p. 1; "The Electrician" Pub. Co., London.
Context: Electric and magnetic forces. May they live for ever, and never be forgot, if only to remind us that the science of electromagnetics, in spite of the abstract nature of its theory, involving quantities whose nature is entirely unknown at the present, is really and truly founded on the observations of real Newtonian forces, electric and magnetic respectively.

“When you expect the best, you release a magnetic force in your mind which by a law of attraction tends to bring the best to you.”

Norman Vincent Peale book The Power of Positive Thinking

Source: The Power of Positive Thinking

“There was a physical quality in us resembling the power of electricity or of the magnet, by which when a pair approached within a striking distance they flew together like the needle to the pole or like two objects in electric experiments.”

John Adams (1735–1826) 2nd President of the United States

As quoted in Complete Book of U.S. Presidents (1984), by William A. DeGregorio, pp. 19–20

“A grocer is drawn to his business by an attracting force quite equal to the repelling force which drives artists away from it.”

Honoré de Balzac (1799–1850) French writer

Source: A Bachelor's Establishment (1842), Ch. I.
Context: A grocer is drawn to his business by an attracting force quite equal to the repelling force which drives artists away from it. We do not sufficiently study the social potentialities which make up the various vocations of life. It would be interesting to know what determines one man to be a stationer rather than a baker; since, in our day, sons are not compelled to follow the calling of their fathers, as they were among the Egyptians.

“Both the law of inertia and the law of gravitation contain a numerical factor or a constant belonging to matter, which is called mass. We have thus two definitions of mass; one by the law of inertia: mass is the ratio between force and acceleration. We may call the mass thus defined the inertial or passive mass, as it is a measure of the resistance offered by matter to a force acting on it. The second is defined by the law of gravitation, and might be called the gravitational or active mass, being a measure of the force exerted by one material body on another. The fact that these two constants or coefficients are the same is, in Newton's system, to be considered as a most remarkable accidental coincidence and was decidedly felt as such by Newton himself. He made experiments to determine the equality of the two masses by swinging a pendulum, of which the bob was hollow and could be filled up with different materials. The force acting on the pendulum is proportional to its active mass, its inertia is proportional to its passive mass, so that the period will depend on the ratio of the passive and the active mass. Consequently the fact that the period of all these different pendulums was the same, proves that this ratio is a constant, and can be made equal to unity by a suitable choice of units, i. e., the inertial and the gravitational mass are the same. These experiments have been repeated in the nineteenth century by Bessel, and in our own times by Eötvös and Zeeman, and the identity of the inertial and the gravitational mass is one of the best ascertained empirical facts in physics-perhaps the best. It follows that the so-called fictitious forces introduced by a motion of the body of reference, such as a rotation, are indistinguishable from real forces…. In Einstein's general theory of relativity there is also no formal theoretical difference, as there was in Newton's system…. the equality of inertial and gravitational mass is no longer an accidental coincidence, but a necessity.”

Willem de Sitter (1872–1934) Dutch cosmologist

p, 125
"The Astronomical Aspect of the Theory of Relativity" (1933)

“Gravitation is not only similar to inertia in its generality, it is also measured by the same number… the mass. The inertial mass is what Newton calls the "quantity of matter": it is a measure for the resistance offered by a body to a force trying to alter its state of motion. It might be called the "passive mass." The gravitational mass, on the other hand, is a measure of the force exerted by the body in attracting other bodies. We might call it the "active" mass. The equality of active and passive, or gravitational and inertial, mass was in Newton's system a most remarkable accidental co-incidence, something like a miracle. Newton himself decidedly felt it as such, and made experiments to verify it, by swinging a pendulum with a hollow bob which could be filled with different materials. The force acting on the pendulum is proportional to its gravitational mass, the inertia to its inertial mass: the period of its swing thus depends on the ratio between these two masses. The fact that the period is always the same therefore proves that the gravitational and inertial masses are equal.”

Willem de Sitter (1872–1934) Dutch cosmologist

Kosmos (1932), Above is Beginning Quote of the Last Chapter: Relativity and Modern Theories of the Universe -->

“What lead me more or less directly to the special theory of relativity was the conviction that the electromotive force acting on a body in motion in a magnetic field was nothing else but an electric field.”

Albert Einstein (1879–1955) German-born physicist and founder of the theory of relativity

Letter to the Michelson Commemorative Meeting of the Cleveland Physics Society (1952), as quoted by R.S.Shankland, Am J Phys 32, 16 (1964), p35, republished in A P French, Special Relativity, ISBN 0177710756
1950s

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