The bridge consists of a system of conductors as shown in fig. 564. The circuit of a constant battery is made to branch at P into two parts, which re-unite at Q, so that part of the current flows through the point M, the other part through the point N. The four conductors A, B, C, D, are spoken of as the _arms_ of the balance or bridge. It is by the proportion existing between the resistances of these arms that the resistance of one of them can be calculated when the resistances of the other three are known. When the current which starts from the battery arrives at P, the pressure will have fallen to a certain value. The pressure in the upper branch falls again to M, and continues to fall to Q. The pressure of the lower branch falls to N, and again falls till it reaches the value at Q. Now if N be the same proportionate distance along the resistances between P and Q, as M is along the resistances of the upper line between P and Q, the pressure will have fallen at N to the same value as it has fallen to at M; or, in other words, if the ratio of the resistance C to the resistance D be equal to the ratio between the resistance A and the resistance B, then M and N will be at equal pressures. To find out if this condition obtain, a sensitive galvanometer is placed in a branch wire between M and N which will show _no_ deflection when M and N are at equal pressure or when the four resistances of the arms "balance" one another by being in proportion, thus: (1) A:C = B:D If, then, the value of A, B, and C be known, D can be calculated. The proportion (1) is reduced to the following equation before substituting. D = BC/A For instance, if A and C be, as in fig. 565, 10 ohms and 100 ohms respectively, and B be 15 ohms, D will be (15 × 100) ÷ 10 = 150 ohms. [Illustration: Fig. 566.--Diagram showing usual arrangement of resistances in arms of Wheatstone's bridge. In practice the bridge is seldom or never made in the lozenge shape of the diagrams, figs. 564 and 565, these being given merely for clearness. The resistance box of fig. 554 is, in itself, a complete "bridge," the appropriate connections being made by screws at various points. The letters in the above diagram correspond with those in figs. 564 and 565, and the three figures should be carefully compared.] As constructed, Wheatstone bridges are provided with some resistance coils in the arms A and C, as well as with a complete set in the arm B. The advantage of this arrangement is that by adjusting A and C, the proportionality between B and D can be determined, and can, in certain cases, be measured to fractions of an ohm. In fig. 565 resistances of 10, 100, and 1,000 ohms are included in the arms A and C.