Varley unit

A unit of electrical resistance, 19th century, standardized at the Electric Telegraph Company in Great Britain, and in concept equal to the resistance of 1 mile of copper telegraph wire. In the 1870's Fleeming Jenkin measured at 25.61 B.A. ohms.¹

The unit was named for C. F. Varley, an engineer working for the company who devised a method of locating faults in underground wires by comparing their resistance to that of good wires.²

One Varley unit (or Varley’s unit) was later taken as equal to 25 Siemens’ units (definition 2), about 23.5 ohms.

1. British Assn. for the Advancement of Science.
Reports of the Committee on Electrical Standards appointed by the British Assn. for the Advancement of Science...With a report to the Royal Society on units of electrical resistance, by Prof. F. Jenkin....
London: E & F. Spon, 1873.

2. Report of the Joint Committee on the Construction of Submarine Telegraphs.
British Parliamentary Papers, 2744 (1860), volume 62, para. 2900 (London, 1861).



Twenty-five units [of the B. A. unit of resistance] are within one per cent. equal to the mile of No. 16 copper wire in use by the Electric and International Company. Mr. Varley has promised that for the future exact equality shall be aimed at.

Third Report - Bath, September 14, 1864.
Reports of the Committee on Electrical Standards Appointed by the British Association for the Advancement of Science.
London: E. and F. N. Spon, 1873.
Footnote, page 112.


L'unité de Varley, 1 mille anglais d'un fil de cuivre particulier, ayant 1/16 pouce de diamètre.

The Varley unit, 1 English mile of wire of a certain copper alloy, being 1/16th of an inch in diameter.

H. Schoentjes.
Les Grandeurs Électriques et leurs Unités. 2nd ed. revised and augmented.
Paris: Librairie de Gauthier-Villars Éditeur, 1884.
Page 102.

For Further Reading

Bruce J. Hunt.
The Ohm Is Where the Art Is: British Telegraph Engineers and the Development of Electrical Standards.
Osiris, volume 9, pages 48-63 (1994).

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