Kelvin double bridge posses and extra ratio are connected in the bridge circuit and hence it is call (DSC). The first ratio are is P & Q the second ratio are p & q is used to connect the golvanometer to a point d at the appropriate potential between points m & n to eliminate the effect of connecting lead of resistance r between the resistance are and standard resistance.
The Kelvin double bridge, also known as the Kelvin Varley bridge, is a specialized type of bridge circuit commonly used for precise resistance measurements. It was developed by the Scottish engineer William Thomson, also known as Lord Kelvin, in the 19th century. The Kelvin double bridge is designed to minimize errors caused by contact resistances and lead wire resistance, allowing for highly accurate resistance measurements.
The Kelvin double bridge is an extension of the basic Wheatstone bridge configuration and incorporates additional ratio arms and measurement techniques to improve accuracy. The bridge consists of four arms or branches: two ratio arms, one containing a variable resistance (R) and an unknown resistance (Rx), and the other containing fixed resistors (R1 and R2). The remaining two arms are used for voltage measurements and are known as the ratio arms.
The primary principle behind the Kelvin double bridge is based on the balance condition, where the ratio of the voltage across the unknown resistance (Rx) to the voltage across the variable resistance (R) is equal to the ratio of the resistors in the ratio arms. When the bridge is balanced, the voltage difference across the galvanometer connected between the two junction points of the ratio arms becomes zero.
The key advantage of the Kelvin double bridge is its ability to minimize errors caused by contact resistances and lead wire resistance. This is achieved by using a four-wire Kelvin connection, where two additional separate connections, known as the Kelvin sense leads, are made to the measurement points of the unknown resistance (Rx). By passing the measuring current through one pair of leads and sensing the voltage across the other pair of leads, the impact of contact resistances and lead wire resistance is effectively eliminated, allowing for accurate measurements of low-resistance values.
The Kelvin double bridge is widely used in applications where high precision resistance measurements are required, such as in metrology laboratories, calibration facilities, and electronic manufacturing. It provides a reliable and accurate method for determining unknown resistance values with minimal errors caused by contact and lead wire resistances.
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