# 4.11: Wheatstone Bridge

\(\text{FIGURE IV.9}\)

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. *R*_{1 }and *R*_{2} can be varied. *R*_{3} is a standard resistance whose value is known. *R*_{4 }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 *R*_{1} and *R*_{2} 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 \(R_4=R_1R_3/R_2\).