## 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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