Page:Electricity (1912) Kapp.djvu/134

130 Thus each conductor has a certain physical property called its electric resistance, and this depends on the length and section of the conductor, on its temperature and on its material. In order to compare different materials as regards resistance, we must eliminate those elements which may vary from case to case and reduce all to the same standard. The physicist takes as the standard length the centimetre, and as the standard cross section the square centimetre. The standard form for which the resistance is given is thus not a wire at all, but a cube. The engineer prefers to retain the shape of the wire for his standard, and defines the resistance of the material as that of a wire one metre long and one square millimetre in cross section, the test being made at the temperature of 15° C.

How is such a test to be made? G. S. Ohm, a Bavarian physicist (1787-1854), was the first to make such tests and to formulate a law, which bears his name, and which connects the three things on which the transfer of electricity from one end of a conductor to the other depends. Ohm found experimentally that the strength of the current is directly proportional to the e.m.f. applied at the ends of the wire, and inversely