Kohlrausch, Rudolph Herrmann Arndt

(b. Göttingen, Germany, 6 November 1809; d. Erlangen, Germany, 9 March 1858)

physics.

Kohlrausch is best remembered for showing, with Wilhelm Weber, that the ratio of the absolute electrostatic unit of charge to the absolute electromagnetic unit of charge equals the speed of light.

Kohlrausch taught mathematics and physics, successively, at the Ritterakademie at Lüneburg, the Gymnasium at Rinteln, the Polytechnikum in Kassel, and the Gymnasium at Marburg . He became professor at the University of Marburg in 1853 and at the University of Erlangen in 1857, a year before his death. Kohlrausch was the father of the physicists Friedrich Wilhelm Kohlrausch and Wilhelm Friedrich Kohlrausch and the grandfather of the physiologist Arnt Ludwig Friedrich Kohlrausch.

Kohlrausch improved the operation of the Dellmann electrometer (1847-1848) and measured the electromotive force of various cells (1849-1853). He verified Ohm’s law in electric circuits in 1848 when he showed that the electromotive force produced by a cell was proportional to the electroscopic tension of the same cell.

In 1856 Kohlrausch and Weber used the tangent galvanometer, developed by Weber, to determine experimentally the electromagnetic value of the discharge current when a Leyden jar is discharged through the galvanometer. They compared this value with the value, determined experimentally, of the electrostatic charge contained in the Leyden jar before discharge. Kohlrausch and Weber found that the ratio of the two measurements—electrostatic to electromagnetic—equalled 3.107 x 10¹⁰ cm. sec., a figure close to the accepted value for the velocity of light. This result was the continuation of Weber’s measurements, with Gauss, of the absolute units of terrestrial magnetism.

The coincidence of the ratio and the speed of light led Kirchhoff to state in 1857 that an electric disturbance was propagated along a perfectly conducting wire at the velocity of light.

BIBLIOGRAPHY

Kohlrausch’s works have not yet appeared in collected form. Original articles can be found in various journals, including Poggendorff’s Annalen der Physik und Chemie, in which the major paper with Weber, “Ueber die Electricitätsmenge, welche bei galvanischen Strömen durch den Querschnitt der Kette fliest,” was published (99 [1856], 10-25). It was reprinted with Friedrich Kohlrausch’s paper on conductivity of solutions, in Ostwald’s Klassiker der exakten Wissenschaften, 142 (1904).

Ollin J. Drennan