Back Reflection
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The History of the Ohm In 2011 (Issue 59), Back Reflection recognised the immortalisation of Michael Faraday and Joseph Henry in the SI units for capacitance and inductance. However, 2014 marks the 160th anniversary of the death of the first and arguably the most well know scientist to have his name linked with an electrical standard, Georg Simon Ohm (1787 – 1854). This association began when Sir Charles Tilston Bright (1832 – 1888) and Josiah Latimer Clark (1822 – 1898) proposed the name “ohma” to be a unit of electromotive force in a paper to the British Association for the Advancement of Science committee. This committee was set up in 1861 to establish standard electrical units, and was established in line with a recommendation in the report from the joint British Government and Atlantic Telegraph Company committee of
by Stewart Ash
enquiry, set up to look into the failures of the Atlantic and Red Sea cables. Resistance as a unit had been expressed as a velocity, based on the ten million metres of the earth quadrant (from which we get the original definition of the metre). This was explained by James Clark Maxwell (1831 – 1897) as “the velocity of a body which, in one second, travels the distance from the pole to the equator measured along the meridian of Paris”. Charles Wheatstone (1802 – 1875) had proposed an arbitrary resistance standard in 1843 based on a foot of copper weighing 100 grains. Another arbitrary standard was based on one mile of copper wire 1/16 inch in diameter. In Europe, the arbitrary standard unit was based on Dr Werner von Siemens (1816 – 1892) mercury filled glass tube. A number of other
arbitrary standards also co-existed, with little or no correlation between any of them. Clark and Bright’s paper entitled “Measurement of Electrical Quantities and Resistance” was presented to the British Association committee led by Professor William Thomson [Later Lord Kelvin] (1824 – 1907) and Maxwell, in Manchester in September 1861. It began: “The Science of Electricity and the art of Telegraphy have both now arrived at a stage of progress at which it is necessary that universally received standards of electrical quantities and resistance should be adopted, in order that precise language and measurement may take the place of the empirical rules and general ideas now prevalent.” By 1864, the British Association (BA) had created the “BA unit” of resistance,