# Carey foster bridge

## Carey foster bridge

Carey foster bridge is the method used for measurement of medium resistances. Carey foster bridge is specially used for the comparison of two equal resistances. A slide wire having length L is included between resistance R and S. When resistance P and Q are adjusted so that the ratio P/Q is approximately equal to R/S. So this can be achieved by sliding contact on slide wire.

The Carey foster bridge figure below.

Let P, Q, R and S are the four resistance used in the bridge. The value of resistances  P and R are known while the R and S are the unknown resistors. The length L (of slide wire)   is placed between the resistance R and S. The resistance P and Q are adjusted so that the ratio of P/Q is equivalent to the R/S. The ratio of the resistance is equivalent by sliding contact on the sliding wiring between R and S resistance.

Let l1 is the distance from the left side at which the balanced is obtained. Now the R and S are interchanged, and at this time the balanced is obtained by sliding the contact at the distance of l2.

equation for first balance:-
r is the resistance of per unit length of slide wire.
After interchanging Resistance S and R, the equation is

equation of the balance bridge:-

Comparing,

Reversible switch is used for interchanging the resistor S and R during the test.Using the Carey foster bridge, the direct comparison between the resistance S and R.The l1 and l2 are the balanced points when slide wire is calibrated . At this condition the resistance of other components like the resistance of P, Q and sliding contact are completely eliminated.

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