1. wheatstone's bridge
A system of connections applied to parallel circuits, including resistance coils for the purpose of measuring an unknown resistance. A single current is made to pass from A through two parallel connected branches, joining together again at C. A cross connection B D has a galvanometer or other current indicator in circuit. In any conductor through which a current is passing, the fall of potential at given points is proportional to the resistance between such points. Referring to the diagram a given fall of potential exists between A and C. The fall between A and B is to the fall between A and C as the resistance r between A and B is to the resistance r + r' between A and C. The same applies to the other branch, with the substitution of the resistances s and S' and the point D for r r' and B. Therefore, if this proportion holds, r : r' : : s : S'. No current will go through B D , and the galvanometer will be unaffected. Assume s' to be of unknown resistance, the above proportion will give it, if r, r' and s are known, or if the ratio of r to r' and the absolute value of s is known.
In use the resistances r, r', and s are made to vary as desired. To measure an unknown resistance it is introduced at S', and one of the other resistances is varied until the galvanometer is unaffected. Then the resistance of S' is determined by calculation as just explained. The artificial resistances may be resistance coils, q. v., or it is enough to have one unknown resistance at s. Then if the length of wire ABC is accurately known, the point B can be shifted along it until the balance is attained. The relative lengths A B, and B C, will then give the ratio r : r' needed for the calculation. This assumes the wire ABC to be of absolutely uniform resistance. This is the principle of the meter-bridge described below. The use of coils is the more common method and is carried out by special resistance boxes, with the connections arranged to carry out the exact principle as explained. The principle of construction and use of a resistance box of the Wheatstone bridge type, as shown in the cut, is described under Box Bridge, q. v.
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Definitions of wheatstone's bridge
