Before the ohm was a formula, it was a tube of mercury
For decades the standard of electrical resistance was a slender glass tube filled with mercury, cut to a precise length and chilled to freezing point. Telegraph engineers needed a common yardstick, and getting everyone to agree on one took congresses in Paris, Chicago and London before physics finally replaced the glassware.
Early telegraphy ran on improvised standards. Resistance was quoted as so many lengths of some agency's reference wire, and because each agency used its own wire, figures did not translate. Worse, none of these units fitted neatly with those for energy or time, so any calculation linking power to resistance needed fudge factors. One quirk of the more rigorous absolute approach was that resistance worked out to have the dimensions of a speed, and one early proposal defined its unit as a quarter of the Earth's circumference per second.
In 1860 Werner Siemens suggested a column of pure mercury one metre long and one square millimetre in section. A year later Latimer Clark and Charles Bright urged the British Association to set electrical standards with names honouring great scientists, and the association formed a committee that included James Clerk Maxwell and William Thomson. Its 1864 report spoke of the B.A. unit, or Ohmad, and by 1867 the word had shortened to ohm. The committee's version turned out 1.3 percent too small because of a calculation slip.
Compromise followed compromise. A Paris conference in 1884 picked a mercury column 106 centimetres long, between the rival values, though no country wrote it into law. Chicago in 1893 recommended an international ohm based on a column 106.3 centimetres long and weighing 14.4521 grams, and London endorsed it in 1908. Mercury reigned until 1948, when the ohm was redefined in absolute terms; glass tubes with uneven bores had always made it hard to reproduce. Since 1990 the quantum Hall effect has anchored it, and the 2019 SI revision tied it to fundamental constants.
The name honours Georg Ohm. William Henry Preece proposed the Greek omega as its symbol in 1867 for the similar sound. Electronics makers often use R instead, so a 5.6 ohm part is printed 5R6, which keeps a faint decimal point from being lost.
Source: Ohm
