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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