OHM (named after Georg S. Ohm). The practical unit of electrical resistance. It is defined as being the electrical resistance offered to the passage of an unvarying electric current by a column of mercury, 106.3 cm. long, of uniform cross-section, having a mass of 14.4521 grams, the mercury being at the temperature of 0° C. (This would require a cross-section of 1.00003 square millimeters.) The ohm is substantially equal to 109 C.G.S. electro-magnetic units of resistance. See Electrical Units; Resistance, Electrical.