The Wheatstone bridge can be used to compare the value of two resistances – or, if the unknown resistance is compared with a resistance whose value is known, it can be used to measure an unknown resistance. R1 and R2 can be varied. R3 is a standard resistance whose value is known. R4 is the unknown resistance whose value is to be determined. G is a galvanometer. This is just a sensitive ammeter, in which the zero-current position has the needle in the middle of the scale; the needle may move one way or the other, depending on which way the current is flowing. The function of the galvanometer is not so much to measure current, but merely to detect whether or not a current is flowing in one direction of another. In use, the resistances R1 and R2 are varied until no current flows in the galvanometer. The bridge is then said to be “balanced” and \(R_1/R_2=R_3/R_4\) and hence the unknown resistance is given by \(R_4=R_1R_3/R_2\).