WHEATSTONE'S BRIDGE. An important and much used method for the measurement of electrical resistance. Employing a battery, a galvanometer, and auxiliary resistances, it is possible to determine the resistance of a con ductor with a high degree of accuracy. The principle of the method will be readily under stood by reference to the accompanying diagram. Let B be a battery whose terminals are con nected with the ends of a straight wire of German ab, which is stretched along a scale. The current flowing from-a to b divides at a, a part of it traversing the bridge wire, as al) is termed. and a part flowing through R and X, the former being a resistance whose value is known and the latter the material whose resistance is to lie measured. The two branch circuits unite at h and return to the battery. At t, the junction of B and X, one terminal of the galvanometer. (4., is attached, while the other, S, is connected with a slider which can be moved along the wire so as to make contact at any desired point. Let the current at a be at a higher potential titan at h, then neeording to Ohm's law there will be a fail of potential along the wire depending upon the resistance, and, since ab is a uniform conductor on the length, this will he proportional to the length. The same fall of potential as along the wire ab must also take place through R and X, consequently some point on ab will be found which is at the same potential as t. Where there is a difference of potential, there must result a flow of electricity which will deflect the needle of the galvanometer. We can determine when the two points are at the same potential by mov ing the sliding contact until there is no deflec tion of the needle on closing the circuit. Con
sequently there must be the same fall of poten tial from a tot and from t to b as from a to S and from S to b. It has been shown that the fall of potential along aS and Sb is proportional to the distances of these parts from the ends of the wire, which we can denote by A and B, therefore the resistance R must be to the. resistance A is to 13. In practice Il is selected so that the measurement will be made when the point of contact is at the centre of the wire rather than at the ends, for, as can be readily seen, the degree of accuracy is far greater when A and B are of nearly the same value than when one is large with respect to the other. The wire bridge, as the apparatus is termed, is useful for many purposes and answers well in explaining the principle. but the apparatus is generally constructed with a series of resistance coils in the form known as the Post-Office box (so called from the fact that the operation of the lines in England is under the control the Post-Office). In this arrangement there is a series of fixed ratios formed by roils of wire taking the place of the stretched wire used above and other coils so arranged as to give I; any value between 1 and 10.000 ohms. The ratio coils generally have their resistances 10, 100, and 1000 ohms, which permits a range of measurements from to 1.000.000 ohms.
See RESISTANCE. ELECTRICAL. Consult: Kempe, Electrical Testing (6th ed., London, 1901) ; Thom pson, Elementary Lessons in Magnetism and Electricity (rev. ed.. New York. 1901).