**GEORG SIMON OHM** (1781-1854), was born at Erlangen in 1781 and educated at the university there. He became professor of mathematics in the Jesuits' college at Cologne in 1817 and in the polytechnic school of Nuremberg in 1833, and in 1852 professor of experimental physics in the university of Munich. He died 6th July 1854.

His writings are numerous, but, with one important exception, not of the first order. The exception is his pamphlet published in Berlin in 1827, with the title *Die galvanische Kette: mathematisch bearbeitet* [The Galvanic Circuit Investigated Mathematically]. This work, the germs of which had appeared during the two preceding years in the journals of Schweigger and Poggendorff, has exerted most important influence on the whole development of the theory and applications of current electricity. Now-adays "Ohm's Law," as it is called, in which all that is most valuable in the pamphlet is summarized, is as universally known as anything in physics. It may be doubted whether Ohm's investigation could have been made but for the magnificent work of Fourier on the Conduction, of Heat. In fact, the equation for the propagation of electricity formed on Ohm's principles is identical with that of Fourier for the propagation of heat; and if, in Fourier's solution of any problem of heat-conduction, we change the word "temperature" to "potential" and write "electric current" instead of " flux of heat," we have the solution of a corresponding problem of electric conduction. The basis of Fourier's work, without which even his splendid mathematical powers would have been of no avail, was his clear conception and definition of conductivity. But this involves an assumption, undoubtedly true for small temperature-gradients, but still an assumption, viz., that, all else being the same, the flux of heat is strictly proportional to the gradient of temperature. An exactly similar assumption is made in the statement of Ohm's law, i.e., that, other things being alike, the strength of the current is at each point proportional to the gradient of potential. It happens, however, that with our modern methods it is much more easy to test the accuracy of the assumption in the case of electricity than in that of heat; and it has accordingly been shown by Maxwell and Chrystal that Ohm's law is true, within the limits of experimental error, even when the currents are so powerful as almost to fuse the conducting wire. The value of Ohm's work was but imperfectly recognized until it was stamped by the award of the Copley medal of the Royal Society in 1841.