The Wheatstone Bridge
An unknown resistance value can be accurately measured using a circuit known as a
Wheatstone bridge (Fig. 28.25). This circuit consists of the unknown resistance Rx ,
three known resistances R1 , R2 , and R3 (where R1 is a calibrated variable resistor),
a galvanometer, and a battery. The known resistor R1 is varied until the galvanometer
reading is zero—that is, until there is no current from a to b. Under this condition the
bridge is said to be balanced. Because the electric potential at point a must equal the
potential at point b when the bridge is balanced, the potential difference across R1 must
equal the potential difference across R2 . Likewise, the potential difference across R3must equal the potential difference across Rx . From these considerations we see that
Dividing Equation (1) by Equation (2) eliminates the currents, and solving for Rx , we find that
21
Top Related