KOHLRAUSCH, FRIEDRICH WILHELM (184o 1910), German physicist, was born on Oct. 14, 1840, at Rinteln on the Weser, the son of Rudolph Kohlrausch (1809-58), who, with Weber, had measured the ratio of the electromagnetic to the electrostatic unit of charge. Kohlrausch studied at Gottingen and Erlangen. He held chairs at Gottingen (1866-7o), at the School of Technology, Frankfurt-on-Main (187o-71), at Darm stadt (1871-75), at Wiirzburg (1875-88), at Strasbourg (1888 95), and finally he succeeded Helmholtz as president of the Reichanstalt at Charlottenburg. In 1900 he was made honorary professor of physics at the University of Berlin. He was a member of the scientific societies of many European countries, including the Royal Society and Physical Society of London. He died at Marburg on Jan. 17, 191o.
His research work was mainly on electricity and magnetism; he devised a method of simultaneously determining the measure ment of the horizontal component of the earth's magnetic field and an electric current. He carried out a series of important in vestigations on electrolytic conductivity, using an alternating current in conjunction with a bridge method which reduced the polarisation of the electrolyte. This method is used to-day and is
called after Kohlrausch. He investigated the variation of con ductivity with dilution and showed that the ratio of the conduc tivity to the number of gram-equivalents of the salt per unit volume approached an upper limit for infinite dilution. He obtained a connection between this ratio and the sum of the velocities of the ions; this with the determination of the ratio of the velocities made by Hitorff enabled the absolute velocities of the ions to be found. Kohlrausch also did much to stimulate experimental work in physics, his Leitfaden der Praktischen Physik (1870) ran into many editions, and was translated into English. He constructed a number of electric and magnetic measuring instruments.
See Obituary Notice by Warburg in Deutsche Physikalische Gesell schaft Verhandlungen, xii. (191o).