The history
From a scientist timing his own electric shocks to an ohm fixed by the constants of nature.
People could make sparks long before they could make a steady current, and they could feel current long before they could measure it. Once Volta's pile gave a steady push and Ørsted's compass gave a way to measure the flow, a schoolteacher in Cologne found the simple rule linking them. It was ignored, then celebrated, then named into a unit. The twentieth century explained why metals obey it, found materials that break it completely, and finally turned the ohm into a constant of nature.
1781
Current found proportional to 'electrification' (unpublished)
Henry Cavendish, England
1800
A steady electric current
Alessandro Volta, Italy
1827
Ohm's law published
Georg Simon Ohm, Germany
1881
International ohm agreed
International Electrical Congress, Paris
1911
Superconductivity observed
Heike Kamerlingh Onnes, Netherlands
1980
Quantum standard of resistance
Klaus von Klitzing, Germany (at Grenoble, France)
1780 – 1824
Scientists learn to make a steady current and to measure it, and notice that long, thin or hot wires conduct worse.
1781
January 1781
With no instrument to measure current, Cavendish sent charge from Leyden jars through tubes of salt water and judged the strength of the shock in his own body. He found the current rose in step with the 'degree of electrification', which is Ohm's law in all but name. He never published it.
Why it mattered. The law was found once and then lost for almost a century, until James Clerk Maxwell edited Cavendish's papers in 1879.
1800
20 March 1800
Volta stacked discs of zinc and copper with brine-soaked cloth between them and described the 'pile' in a letter to the Royal Society in London. Unlike a Leyden jar, which empties in a spark, it pushed a steady current for as long as the chemicals lasted.
Why it mattered. Without a steady current, no one could measure how current depends on anything.
1820
Ørsted showed that a wire carrying current swings a nearby compass needle, and published it in July 1820. Within months others turned the effect into the galvanometer: the stronger the current, the further the needle turns.
Why it mattered. It gave scientists the first way to measure current, which Ohm would use.
1821
Davy found that a wire conducts less the longer it is and more the thicker it is, and that metals conduct worse as they get hotter. He ranked silver first, then copper.
Why it mattered. These are the rules behind R = ρL ÷ A, and behind the hot bulb filament of chapter 5.
1821
1821–1822
Seebeck found that a loop of two different metals makes a current when one junction is hotter than the other. The push depends only on the temperature difference, so it is very steady.
Why it mattered. Ohm's first batteries drifted too much. A thermocouple with one end in ice and one in boiling water gave him a push he could trust.
1825 – 1845
A Cologne schoolteacher finds the law, is ignored, then wins the Royal Society's top prize. Joule, Wheatstone and Kirchhoff build on it.
1825
Ohm taught mathematics and physics at the Jesuit Gymnasium in Cologne. In his spare time he measured how the magnetic effect of a current weakens as the wire gets longer, and published his first paper on it in 1825.
Why it mattered. He was an outsider with a school lab, not a university professor.
1826
Using a bismuth–copper thermocouple as a steady source and a torsion galvanometer as his meter, Ohm tested wires of different lengths. His readings fitted a simple rule, x = a ÷ (b + l): the current falls as the wire's length l is added to the circuit's own resistance b.
Why it mattered. It is Ohm's law in the form 'current = push ÷ total resistance'.
1827
Ohm published his theory as a book, The Galvanic Circuit Investigated Mathematically, modelled on Fourier's mathematics of heat flow. It was coldly received. One critic called it a web of naked fancies, and Ohm left Cologne for years of poorly paid work.
Why it mattered. A correct law can be ignored when it arrives as maths from an unknown teacher.
1840
1840–1841
Joule, a brewer's son, measured the heat from wires carrying current. He found it was proportional to the resistance times the square of the current: I² × R. He sent the work to the Royal Society in 1840 and published it in full in 1841.
Why it mattered. Joule's law is Ohm's law's partner: it explains every heater, fuse and warm cable.
1843
Christie devised it in 1833; Wheatstone improved it and made it famous in 1843. Four resistances form a diamond with a galvanometer across the middle. When the needle reads zero, the unknown resistance equals a simple ratio of the others.
Why it mattered. It made resistance measurable with great precision, which telegraph engineers badly needed.
1845
While still a student, Kirchhoff generalised Ohm's work into two rules: the currents into any junction add up to the currents out, and the voltages round any loop add up to zero.
Why it mattered. With Ohm's law and Kirchhoff's rules you can work out any network of resistors.
1860 – 1893
Telegraph engineers need standard units. The ohm is named, defined and agreed between nations.
1861
1861–1864
Clark and Bright proposed naming electrical units after scientists, including the 'ohma'. The British Association set up a committee with Maxwell and William Thomson, and in 1864 it defined the 'B.A. unit' of resistance, later found to be about 1.3% too small.
Why it mattered. Electricity got a shared language, and Ohm's name became a unit.
1881
Scientists from many nations agreed on the practical units of electricity, including the ohm. For decades after, the ohm was realised as a column of mercury about 106 cm long at 0 °C.
Why it mattered. A resistor made in one country could now be trusted in another.
1900 – 1990
Physicists explain Ohm's law with drifting electrons, then find superconductors, where resistance vanishes.
1900
Drude pictured a metal as a gas of free electrons that bump into the atoms. An electric field gives them a slow drift on top of their random motion, and the drift speed grows in proportion to the field. That gives J = σE, Ohm's law for a piece of material.
Why it mattered. Ohm's law stopped being just a rule and got a reason.
1911
8 April 1911
Having liquefied helium, Onnes's lab cooled solid mercury and measured its resistance. At about 4.2 K it abruptly dropped to zero. He won the 1913 Nobel Prize for his low-temperature work.
Why it mattered. Superconductors break Ohm's law completely: current flows with no voltage at all.
1986
1986–1987
At IBM Zurich, Bednorz and Müller found a copper-oxide ceramic that superconducts at about 35 K. Within a year Chu's group made YBCO, which works at about 93 K, above the boiling point of cheap liquid nitrogen.
Why it mattered. Zero-resistance wires moved from rare physics labs towards MRI machines and power cables.
By the numbers
The temperature below which the best known material loses all resistance. Liquid nitrogen boils at 77 K, so the jump in 1987 made superconductors far cheaper to cool.
1980 – today
A quantum effect gives an exact resistance, and the ohm becomes a constant of nature.
1980
February 1980
In very thin layers of electrons, cold and in a strong magnetic field, von Klitzing found the Hall resistance locks onto exact steps of h/e², about 25,812.807 Ω, whatever the sample. He won the 1985 Nobel Prize, and from 1990 labs used it to calibrate resistance.
Why it mattered. The best standard of resistance became a law of nature, not a column of mercury.
2019
20 May 2019
The SI was redefined so that Planck's constant h and the electron's charge e have exact values. That makes the von Klitzing constant h/e² exact too, so the ohm is now defined by constants of nature.
Why it mattered. Two centuries after a schoolteacher's wires, the ohm is built into the definition of all units.
Did you know?
Electrons in a household wire drift at well under a millimetre per second, but the push that moves them travels at about two thirds of the speed of light.
Before any meter existed, Henry Cavendish measured current by how much the shock hurt.
A 60 W bulb's filament has about 14 times more resistance hot than cold, which is why bulbs usually blow at switch-on.
Since 2019 the ohm has been defined through Planck's constant and the charge of the electron.
Dry skin can resist about 100,000 Ω; wet skin can fall to about 1,000 Ω, which is why water and electricity don't mix.
The people
1789 – 1854 · Physicist and teacher · Germany
Found the law as a schoolteacher in Cologne; professor in Munich from 1852.
1731 – 1810 · Natural philosopher · England
Found the law decades earlier using his own body as the meter, and kept it to himself.
1745 – 1827 · Physicist · Italy
Built the first battery; the volt is named after him.
1777 – 1851 · Physicist and chemist · Denmark
Showed that current moves a compass needle, the basis of the first current meters.
1818 – 1889 · Physicist and brewer · England
Showed that a wire's heat equals I² × R.
1853 – 1926 · Physicist · Netherlands
Liquefied helium and discovered superconductivity.
born 1943 · Physicist · Germany
Discovered the quantum Hall effect, now the world's resistance standard.
Where it happened
Sources
Dates marked “c.” are approximate, and historians sometimes disagree about who was first. If you spot a mistake, tell us.
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