John Michell was a polymath: the first person in history to suggest black holes could exist; the first to suggest that earthquakes are caused by movements of rocks miles below the earth’s surface; and the first to suggest that the force between two magnets is governed by an inverse square law.
He invented the torsion balance to weigh our planet. He used probability theory to reveal that some star groupings are non-random and therefore perhaps held together by gravity.
Achievements and Key Points
- Made the first explicit statement that the force between two magnets is governed by an inverse square law.
- Suggested that black holes – he called them dark stars – could exist, proposing their masses could be so great that light could not escape from them.
- Proposed that earthquakes are caused by the movement of rocks miles below our planet’s surface and travel long distances as waves.
- Invented the torsion balance, allowing our planet’s mass to be determined for the first time.
- Used probability analysis to show that some star clusters, such as the Pleiades, are non-random, suggesting they are held together by gravity.
- Invented a method to produce cheap, permanent magnets artificially.
Michell’s Lifetime in Context
John Michell was born on December 25, 1724 in the village of Eakring, Nottinghamshire, England, UK. His mother was Obedience Gerrard. His father was Gilbert Michell, a Church of England priest. John was the eldest of their three children, all of whom were schooled at home to a very high standard.
Michell, who seems to have had an early interest in mathematics, chose to go to the University of Cambridge, beginning in June 1742, age 17. The university was famous for Isaac Newton’s epoch-changing breakthroughs in the late 1600s. It was also the obvious choice for anyone interested in mathematical sciences because of its interpretation of the law.
The law said university undergraduate courses had to consist of two years of Logic and one of Philosophy. Cambridge said that Mathematics was Logic and taught it for two years. Instead of General Philosophy, there were classes in Natural Philosophy (Science). Much of the curriculum actually consisted of Newtonian Mathematics and Newtonian Natural Philosophy. Michell mixed his studies with paid employment at the university and obtained a bachelor’s degree with first class honors in 1748, age 23.
Cambridge Academic Career
The following year, Michell was elected a fellow of Queens’ College, Cambridge, where he lectured for 15 years, teaching subjects as diverse at Geometry, Hebrew, Theology, and Greek. He also took further degrees including, at age 36, a Bachelor of Divinity in 1761. In his final two years he was a Geology professor.
Michell stayed at Cambridge for a total of 21 years. He left in 1763 to become a clergyman. He left Cambridge because fellows of the university were required to be single and he wanted to get married.
The Science of John Michell
Inverse Square Law of Magnetism
In 1750, Michell published a short book: A Treatise of Artificial Magnets.
In it he said an inverse square relationship governs the forces of attraction and repulsion between the poles of different magnets. So, for example, if you double the distance between magnets, the force between them decreases by a factor of four.
Michell said he made the claim on the basis of his experiments, but more work was needed:
The credit for the discovery of the inverse-square law for both magnets and electric charges is usually given to Charles-Augustin de Coulomb, who published his work in 1785.
In 1750, when magnetic needles were needed, they were made from magnetite, a naturally occurring iron oxide. Magnetic needles were used, for example, in navigation at sea and on land; miners used them to search for veins of iron; and surveyors used them to guide tunnel construction. They were expensive.
Michell discovered that by stroking strips of steel with existing magnets he could create permanent magnets that were stronger than the original magnets. Magnets produced in this way were much cheaper than naturally occurring magnets.
In 1759, after studying reports of the devastating earthquake and tsunami which destroyed Portugal’s capital city Lisbon in 1755, Michell wrote his own report. He did not confine himself to the Lisbon quake. He described common features of earthquake prone regions all over the world.
In a time when the scientific understanding of earthquakes was extremely poor, Michell made some real progress.
He noted that major shocks are always followed by aftershocks.
He was the first person in history to propose that earthquakes are caused by “shifting masses of rock miles below the surface.” He was also the first to write that earthquakes can travel long distances as waves:
He observed that the source of the Lisbon earthquake seemed to have been under the sea, resulting in the tsunami that struck the city adding to its destruction. (The bay emptied of water, which then came rushing back in, in a huge flood.)
Michell also observed that earthquakes were the likely source of faults in rock strata.
Stars Are Not Distributed Randomly
In 1764, after his wedding, Michell left academia to work as a clergyman. In his spare time, he made remarkable progress in astronomy and physics.
In 1767, he published his analysis of star groupings. Michell applied probability theory to the stars in the Pleiades star cluster. He concluded that the likelihood of this group being produced by chance was only one chance in 496,000. He suggested the stars of the Pleiades were truly associated with one another.
Until Michell published his work, double stars had been noted, but thought of as appearing close to one another in the sky by chance. Michell showed it was highly unlikely they were paired by chance. Probability theory indicated they truly were pairs, held together by gravity.
Michell’s work spurred William Herschel, discover of Uranus, and his sister Caroline to construct their famous catalog of binary stars. In 1804, Herschel proved that binary stars are true companions orbiting one another.
Not Just a Theoretical Astronomer
Michell built a large telescope (10-foot focal length, 30-inch mirror reflector) to make observations of the sky from his garden. It was a superb instrument. After Michell’s death, William Herschel, who made the finest telescopes in the world, bought it.
Weighing the Earth
Michell realized how he could determine the earth’s mass. He invented and built a torsion balance. This was independent of Charles-Augustin de Coulomb’s 1777 construction of a torsion balance.
Unfortunately, Michell died before he could measure our planet’s mass.
After Michell’s death, his good friend Henry Cavendish received Michell’s torsion balance. Cavendish rebuilt the balance, and in 1797-1798 carried out the famous experiments that weighed the world for the first time.
Cavendish found Earth’s density was 5.448 ± 0.038 times that of water. This is only about 1 percent higher than the modern value of 5.513 g cm -3. Since our planet’s volume was already known, the measurement of its density allowed its mass to be determined accurately for the first time.
Prediction of Black Holes
In 1783, Michell suggested that black holes – he called them dark stars – could exist.
These were stars so massive that their gravitational pull resulted in an escape velocity higher than the speed of light. If such stars existed they would naturally be dark. Today we describe such stars as black holes.
Not only did Michell predict the possible existence of black holes, he also told us how they could be detected, writing that we would have to look for their gravitational effects on other nearby stars whose light could be observed.
Family and The End
In August 1764, age 39, Michell married Sarah Williamson, a wealthy young woman. Sarah died the following year, seven weeks after giving birth to their daughter Mary.
In 1767, Michell became rector of St. Michael’s Church of Thornhill, a village in Yorkshire, where he worked as a clergyman and amateur scientist for the rest of his life,
In February 1773, age 48, Michell married Ann Brecknock. They had no children.
Michell was a talented violinist and enjoyed inviting other scientists and philosophers to small parties at his home in Thornhill. Guests included his good friends Henry Cavendish and Joseph Priestley.
John Michell died age 68 on April 29, 1793 in Thornhill, Yorkshire. He was buried in the village cemetery. He was survived by his daughter Mary and his wife Ann.
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A Treatise of Artificial Magnets
Joseph Bentham, 1751
Conjectures concerning the Cause, and Observations upon the Phaenomena of Earthquakes
Phil. Trans. Vol. 51, pp. 566-634, 1 January 1759
An inquiry into the probable parallax and magnitude of the fixed stars from the quantity of light which they afford us, and the particular circumstances of their situation.
Philosophical Transaction of the Royal Society, Vol. 57, pp 234-264, 1767
Sir Archibald Geike
Memoir of John Michell
Cambridge University Press, 1918
David W. Hughes and Susan Cartwright
John Michell, the Pleiades, and Odds of 496,000 to 1
Journal of Astronomical History and Heritage, Vol. 10, No. 2, pp 93-99, 2007
Weighing the World: The Reverend John Michell of Thornhill
Springer Science & Business Media, 2011
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