RESISTANCE, MEASUREMENT OF. That bodies offer resistance to the passage of the electric current through them is shown by the heat developed when the current passes. This heat production was made the basis of a method of comparison by Henley (1774) and Nairne (1780) who concluded from his experi ments that "iron wire resists the passage of the electric fluid much more than copper." (Phil. Trans. [Hutton] z4 p. 688.) Davy (Phil. Trans., 1821, P. showed that the conducting power of wires is proportional to their cross-sectional area divided by their length, but exact ideas were lacking until the importance of Ohm's Law (1827) was appreciated. An absolute unit of resistance could then be defined as the resistance of a conductor in which unit potential difference produces unit current.
The practical unit of resistance is the International Standard Ohm defined (1894 and 1908) as "the resistance offered to an invariable electric current by a column of mercury at the tempera ture of melting ice, 14.4521 grammes in mass, of a constant cross
sectional area, and a length of 106.300 centimetres." This unit is equal to about 1.0005 X absolute C.G.S. units. The megohm is one million ohms, and the microhm one millionth of an ohm. Specific resistance, or resistivity, is defined by p in the equation and is therefore measured in ohm-cm.
The practical measurement of resistivity involves many proc esses and instruments (see INSTRUMENTS, ELECTRICAL) ; but the methods employed may be classified as Comparison Methods and Absolute Methods. In the former a comparison is effected between the resistance of the material in a known form and some standard resistance, in the latter, the resistivity is determined with reference to the fundamental units of length, mass and time. Special methods are required to determine the resistance of in sulating materials and of electrolytes. (See INSTRUMENTS, ELEC TRICAL; ELECTROLYSIS; ELECTRICITY, CONDUCTION OF: Solids.)