WHEATSTONE'S BRIDGE, a network of six conduc tors of which four, AB, BC, CD and DA form a closed circuit while the others, AC and BD, contain a battery and a galva nometer respectively. When the resistances of the first four are so adjusted that no current from the battery in AC flows through the conductor BD containing the galvanometer it follows that the resistance of AB : that of BC : : the resistance of AD : that of DC. Hence the resistance of DC is determined if the ratio of the first pair (the "ratio arms") is known together with the exact magnitude of the third (AD). This is the principle of the
Post Office Box, the Metre Bridge, the Carey Foster Bridge, the Callendar-Griffiths Bridge, and of many other devices for the comparison of resistances, inductances, and capacitances. The arrangement was devised by S. H. Christie in 1833, and used by Sir Charles Wheatstone in 1847 (Scientific Papers, p. 129, or Phil. Trans. 1847). (See INSTRUMENTS, ELECTRICAL.)