Jacobi's unit

Portrait of Moritz Hermann von Jacobi

In the 19ᵗʰ century, two units used in investigations of electricity, both defined by the German physicist Moritz Hermann von Jacobi (1801-1875):


Jacobi’s unit of current defines the strength of an electric current by the quantity of oxygen plus hydrogen gas generated by the electrolysis of water under specified conditions. One Jacobi's unit is the strength of an electric current which in 1 minute yields 1 cubic centimeter of mixed gas, measured at a temperature of 0°C and a pressure of 760 millimeters of mercury. It is approximately 0.09657 ampere.¹

In German, this unit is called the Jacobische Knallgaseinheit. There is no simple English equivalent for the German word Knallgas, literally “explosion gas,” an apt name for a 1:2 mixture of oxygen and hydrogen.

1. Ganot's Physics, page 797.


Unité de Jacobi. - Un courant continu et constant produisant dans un voltamètre 1 centimètre cube de gaz mélangés par minute à la température de 0° et à la pression de 760 millimètres de mercure, à une intensité de un Jacobi.

Jacobi's unit.— A continuous and constant current producing one cubic centimetre of mixed gas per session in a voltmeter at temperature 0° C., and pressure 760 millimetres of mercury.

Édouard Hospitalier.
Formulaire Pratique de l'Electricien.
Paris: G. Masson, 1883.
Page 41.  Translation (1884) by Gordon Wigan.


Jacobi’s unit of resistance is the resistance of 1 meter of wire with a circular cross section, 1 millimeter in diameter, and made of a particular copper alloy. M. H. Jacobi sent lengths of wire representing this unit to numerous prominent scientists in 1848. According to Fleeming Jenkin's measurement, it was about 0.6367 of a B.A. ohm

2. 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.



L'unité de Jacobi, 25 pieds (8m,12) d'un fil de cuivre pesant 345 grains (22gr,4).

Jacobi's unit, 25 pieds (8.12 meters) of a copper wire weighing 345 grains (22.4 grams).

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

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