Carry Foster Bridge
CAREY FOSTER BRIDGE
Introduction
The Carey Foster bridge is an electrical circuit that can be used to measure very small resistances. It works on the same principle as Wheatstone’s bridge, which consists of four resistances, P, Q, R and S that are connected to each other as shown in the circuit diagram in Figure 1. In this circuit, G is a galvanometer, E is a lead accumulator, and K1 and K are the galvanometer key and the battery key respectively. If the values of the resistances are adjusted so that no current flows through the galvanometer, then if any three of the resistances P, Q, R and S are known, the fourth unknown resistance can be determined by using the relationship P R = Q S (1)
Figure 1: Wheatstone’s bridge You may be familiar with the post office box and the meter bridge, which also work on the same principle as Wheatstone’s bridge. In the meter bridge, two of the resistors, R and S, say, are replaced by a one meter length of resistance wire, with uniform cross-sectional area fixed on a meter scale. Point D is an electrical contact that can be moved along the
wire, thus varying the magnitudes of resistances R and S. The Carey Foster bridge is a modified form of the meter bridge in which the effective length of the wire is considerably increased by connecting a resistance in series with each end of the wire. This increases the accuracy of the bridge. While performing this experiment you will balance the Carey Foster bridge by a null deflection method using a galvanometer. You will first determine the resistance per unit length of the material used for the bridge wire, and will then determine the value of an unknown resistance.
Apparatus
• • • • • • • • •
Carey Foster bridge two equal resistances of about 2 ohms each thick copper strip fractional resistance box lead accumulator galvanometer unknown low resistance one way key connecting wires
lead accumulator
fractional resistance box
standard resistances
bridge wire
Introduction
The Carey Foster bridge is an electrical circuit that can be used to measure very small resistances. It works on the same principle as Wheatstone’s bridge, which consists of four resistances, P, Q, R and S that are connected to each other as shown in the circuit diagram in Figure 1. In this circuit, G is a galvanometer, E is a lead accumulator, and K1 and K are the galvanometer key and the battery key respectively. If the values of the resistances are adjusted so that no current flows through the galvanometer, then if any three of the resistances P, Q, R and S are known, the fourth unknown resistance can be determined by using the relationship P R = Q S (1)
Figure 1: Wheatstone’s bridge You may be familiar with the post office box and the meter bridge, which also work on the same principle as Wheatstone’s bridge. In the meter bridge, two of the resistors, R and S, say, are replaced by a one meter length of resistance wire, with uniform cross-sectional area fixed on a meter scale. Point D is an electrical contact that can be moved along the
wire, thus varying the magnitudes of resistances R and S. The Carey Foster bridge is a modified form of the meter bridge in which the effective length of the wire is considerably increased by connecting a resistance in series with each end of the wire. This increases the accuracy of the bridge. While performing this experiment you will balance the Carey Foster bridge by a null deflection method using a galvanometer. You will first determine the resistance per unit length of the material used for the bridge wire, and will then determine the value of an unknown resistance.
Apparatus
• • • • • • • • •
Carey Foster bridge two equal resistances of about 2 ohms each thick copper strip fractional resistance box lead accumulator galvanometer unknown low resistance one way key connecting wires
lead accumulator
fractional resistance box
standard resistances
bridge wire