John Michell (December 25, 1724 – April 29, 1793) was an
EnglishEngland is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west and the North Sea to the east, with the English Channel to the south separating it from continental...
natural philosopher and
geologistFor other uses, see Geologist .A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth and terrestrial planets...
whose work spanned a wide range of subjects from
astronomyAstronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
to
geologyGeology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed...
,
opticsOptics is the branch of physics which studies the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
, and
gravitationGravitation is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass. Gravitation causes dispersed matter to coalesce, thus accounting for the existence of the Earth, the...
. He was both a theorist and an
experimentIn scientific research, an experiment is a method of investigating causal relationships among variables, or to test a hypothesis. An experiment is a cornerstone of the empirical approach to acquiring data about the world and is used in both natural sciences and social sciences...
er.
Michell was educated at
Queens' College, CambridgeQueens' College is a constituent college of the University of Cambridge, England.The college was first founded in 1448 by Margaret of Anjou , and refounded in 1465 by Elizabeth Woodville...
and later became a Fellow of Queens'. He obtained his M.A. in 1752 and B.D. in 1761. In 1760 he was elected a Fellow of the
Royal SocietyThe Royal Society of London for the Improvement of Natural Knowledge, known simply as the Royal Society, or even the Royal, is a learned society for science that was founded in 1660 and is considered by most to be the oldest such society still in existence...
, in the same year as
Henry CavendishHenry Cavendish, FRS was a British scientist noted for his discovery of hydrogen or what he called "inflammable air". He described the density of inflammable air, which formed water on combustion, in a 1766 paper "On Factitious Airs". Antoine Lavoisier later reproduced Cavendish's experiment and...
. In 1762 he was appointed
Woodwardian Professor of GeologyThe Woodwardian Professor of Geology is a professorship at the University of Cambridge. It was founded by John Woodward in 1728. Woodward's will dictated that the professor would be elected by the Archbishop of Canterbury, the Bishop of Ely, the President of the Royal Society, the President of the...
, and in 1767 he became rector of
Thornhill, West YorkshireThornhill, is a village in West Yorkshire, England. It is located on a hill on the south side of the River Calder, and has extensive views of Dewsbury, Ossett and Wakefield...
, near
DewsburyDewsbury is a market town within the Metropolitan Borough of Kirklees, in West Yorkshire, England. It is to the west of Wakefield, and lies by the River Calder and the Calder and Hebble Navigation....
, where he died.
He was thus described by a contemporary commentator:
John Michell, BD is a little short Man, of a black Complexion, and fat; but having no Acquaintance with him, can say little of him. I think he had the care of St. Botolph’s Church CambridgeThe city of Cambridge is a university town and the administrative centre of the county of Cambridgeshire, England. It lies in East Anglia about north of London. It is also at the heart of the high-technology centre known as Silicon Fen....
, while he continued Fellow of Queen's College, where he was esteemed a very ingenious Man, and an excellent Philosopher. He has published some things in that way, on the MagnetIn physics, the term magnetism is used to describe how materials respond on the microscopic level to an applied magnetic field; to categorize the magnetic phase of a material. For example, the most well known form of magnetism is ferromagnetism such that some ferromagnetic materials produce their...
and ElectricityElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
.’
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Gravity, magnetism & light
Michell conceived, sometime before 1783, the experiment now known as the
Cavendish experimentThe Cavendish experiment, done in 1797–98 by British scientist Henry Cavendish, was the first experiment to measure the force of gravity between masses in the laboratory, and the first to yield accurate values for the gravitational constant and the mass of the Earth. However, these were...
. It was the first to measure the force of gravity between masses in the laboratory and produced the first accurate values for the mass of the Earth and the
gravitational constantThe gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...
. He invented and built, independently of co-inventor Charles Augustin de Coulomb, a torsion balance for the experiment but didn’t live to put it to use. His apparatus passed to
Henry CavendishHenry Cavendish, FRS was a British scientist noted for his discovery of hydrogen or what he called "inflammable air". He described the density of inflammable air, which formed water on combustion, in a 1766 paper "On Factitious Airs". Antoine Lavoisier later reproduced Cavendish's experiment and...
, who performed the experiment in 1798. In 1987, gravity researcher A.H. Cook wrote:
- The most important advance in experiments on gravitation and other delicate measurements was the introduction of the torsion balance by Michell and its use by Cavendish. It has been the basis of all the most significant experiments on gravitation ever since.
In 1750 he published at Cambridge a work of some eighty pages entitled
A Treatise of Artificial Magnets, in which is shown an easy and expeditious method of making them superior to the best natural ones. Besides the description of the method of magnetization which still bears his name, this work contains a variety of accurate magnetic observations, and is distinguished by a lucid exposition of the nature of
magnetic inductionElectromagnetic induction is the production of voltage across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field....
.
At one point, Michell attempted to measure the
radiation pressureRadiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
of light by focusing sunlight onto one side of a compass needle. The experiment was not a success: the needle melted.
Geology
In scientific biographies written during the early 20th century, Michell's historical importance is ascribed to his work on geology. His most important geological essay was entitled "Conjectures concerning the Cause and Observations upon the Phaenomena of Earthquakes" (
Philosophical Transactions, li. 1760), which showed a remarkable knowledge of the strata in various parts of England and abroad.
Effect of gravity on light
More recently, Michell's main "claim to fame" is considered to be his letter to Cavendish, published in 1784, on the effect of gravity on light. This paper was only generally "rediscovered" in the 1970s and is now recognised as anticipating several astronomical ideas that had been considered to be 20th century innovations. Michell is now credited with being the first to study the case of a heavenly object massive enough to prevent light from escaping (the concept of
escape velocityIn physics, escape velocity is the speed where the kinetic energy of an object is equal to the magnitude of its gravitational potential energy, as calculated by the equation,...
was well known at the time). Such an object would not be directly visible, but could be identified by the motions of a companion star if it was part of a binary system. Michell also suggested using a
prismIn optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
to measure the gravitational weakening of starlight due to the surface gravity of the source ("gravitational shift"). Michell acknowledged that some of these ideas were not technically practical at the time, but wrote that he hoped they would be useful to future generations. By the time that Michell's paper was "resurrected" nearly two centuries later, these ideas had been reinvented by others.
The mathematician
Pierre-Simon LaplacePierre-Simon, marquis de Laplace was a French mathematician and astronomer whose work was pivotal to the development of mathematical astronomy and statistics. He summarized and extended the work of his predecessors in his five volume Mécanique Céleste...
suggested the same idea of high-gravity objects trapping light in his book
Exposition du Systeme du Monde in 1796. This sort of high-gravity object under Newtonian theory is commonly referred to as a
dark starA dark star is a theoretical object compatible with Newtonian mechanics that, due to its large mass, has a surface escape velocity that equals or exceeds the speed of light...
, and can be thought of as being the predecessor of the modern idea of a
black holeIn general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, not even light, can escape. The black hole has a one-way surface, called an event horizon, into which objects can fall, but out of which nothing can come...
under
general relativityGeneral relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics. It unifies special relativity and Newton's law of universal gravitation, and describes gravity as a...
.
Some of Michell's contributions
- Observations On the Comet of January 1760 at Cambridge, Philosophical Transactions (1760)
- Conjectures Concerning the Cause and Observations upon the Phaenomena of Earthquakes, ibid. (1760)
- A Recommendation of Hadley's Quadrant for Surveying, ibid. (1765)
- Proposal of a Method for measuring Degrees of Longitude upon Parallels of the Equator, ibid. (1766)
- An Inquiry into the Probable Parallax and Magnitude of the Fixed Stars, ibid. (1767)
- On the Twinkling of the Fixed Stars, ibid. (1767)
- On the Means of Discovering the Distance, Magnitude, &c., of the Fixed Stars, ibid. (1784).
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