Question

Explain the principle of Wheatstone bridge and its applications.

Answer 1

The Wheatstone bridge is a circuit used to measure an unknown electrical resistance by balancing it against known resistances. It was invented by Samuel Hunter Christie in 1833 and later popularized by Sir Charles Wheatstone in the mid-19th century. The principle of the Wheatstone bridge is based on the concept of a balanced bridge circuit, where the ratio of resistances in the arms of the bridge determines the unknown resistance value.

The Wheatstone bridge consists of four resistors arranged in a diamond-shaped formation, with an electrical source (e.g., a battery) connected across the top and bottom of the bridge. The unknown resistor, whose resistance needs to be measured, is placed in one of the arms, while the other three resistors are known and have precise values. The basic Wheatstone bridge circuit is as follows:

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        R1
 -------/\/\/\-------
|                     |
|                     |
R2                 Rx (Unknown Resistance)
|                     |
|                     |
 -------/\/\/\-------
        R3


To find the value of the unknown resistance Rx, the bridge circuit is adjusted until there is no current flow through the galvanometer (a sensitive current measuring device) connected between the two central junctions. When the bridge is balanced, the ratio of resistances on one side of the bridge is equal to the ratio on the other side:

R1 / R2 = R3 / Rx

By knowing the values of R1, R2, and R3, we can calculate the unknown resistance Rx. This balance condition is achieved by varying the values of R1, R2, and R3 or by using a variable resistor in one of the arms.

Applications of Wheatstone bridge:

Resistance measurement: The primary application of the Wheatstone bridge is to accurately measure unknown resistances, especially when the resistance is too high or too low to be measured directly with a multimeter. It is commonly used in laboratories and industrial settings for this purpose.

Strain gauge circuits: Wheatstone bridges are used in strain gauge circuits to measure small changes in resistance due to mechanical strain or deformation. These circuits find applications in load cells, pressure sensors, and other devices used for measuring force or pressure.

Temperature measurement: Thermistors and resistance temperature detectors (RTDs) are often used as temperature sensors in Wheatstone bridge configurations. The bridge output can be calibrated to provide an accurate measurement of temperature.

Sensing applications: The Wheatstone bridge is used in various sensing applications, including humidity sensors, gas sensors, and light-sensitive resistors (LDRs). These sensors rely on changes in resistance based on the quantity being sensed.

Circuit calibration: The Wheatstone bridge can be employed to calibrate other circuits, such as potentiometers or variable resistors, to determine their precise resistance values.

Overall, the Wheatstone bridge is a versatile and widely used circuit in electrical measurements and sensing applications, providing accurate and reliable results for various unknown resistance measurements.