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It is essential to keep track of the minutest measurements of voltage, current and resistance in circuits because the slightest error, such as minute resistances in the wrong place, can lead to mistakes in calculations and physical applications of devices. This is where the Wheatstone Bridge comes in.
The Wheatstone Bridge is a simple circuit used for measuring an unknown resistance by comparing its value with that of known resistance. While this circuit was originally built simply to detect current, its usage has now increased. It is now used to detect and measure small resistance changes. Thus, it finds applications in several devices such as transducers, sensors and thermistors.
The Wheatstone bridge was invented in 1833 by Samuel Hunter Christie, a physicist and mathematician. However, it was popularised only in 1843 when Sir Charles Wheatstone improved the circuit and made it more efficient.
What is Wheatstone Bridge Definition?
The Wheatstone Bridge is a basic circuit used for measuring unknown resistance in small amounts. The measurement of resistance is done by connecting the resistor with three resistors of known value in a quadrilateral arrangement. Then a voltage is applied between one pair of opposite corners and calculations are made.
Construction of the Wheatstone Bridge
The Wheatstone Bridge circuit is constructed with two serial and two parallel resistances, connected through voltage supply and ground terminals. The circuit thus consists of two input and two output terminals along with four resistors that are placed to form a diamond shape.
- The Wheatstone Bridge consists of four arms with four resistances connected (P, Q, R and S).
- The points ac are connected with a source emf (E) and points bd are connected with a galvanometer, also known as a null detector.
- The resistances P and R are in series connection with each other, and so are S and Q. But the combination of P and R resistances is in a parallel connection with resistances S and Q.
- The value of resistance through P and Q is known. S is standard resistance, whereas R indicates the unknown resistance to be measured.
- Currents I1, I3, I2 and I4 flow through resistors P, Q, R and S, respectively.
- When the ratio of these resistances is equal, no current flows through the galvanometer. This is said to be the balanced condition of the Wheatstone Bridge.
- On the other hand, a potential difference between points b and d causes a current to flow through the galvanometer. This is called the unstable condition of the Wheatstone bridge.
Wheatstone Bridge Formula
As stated above, during the balanced condition of the Wheatstone Bridge, the ratio of resistance is equal. This can be shown in mathematical terms as below:
The above shown equation is called the Wheatstone Bridge formula.
Wheatstone Bridge Sensitivity
When the Wheatstone Bridge is in balanced condition, the galvanometer shows zero current. However, the reading deflects with the smallest unbalance in the bridge circuit. It can be thus implied that the deflection of the galvanometer depends upon its sensitivity.
This can be indicated in mathematical terms as:
Here,
Sg is the sensitivity of the galvanometer
is the deflection of the galvanometer
Ig is the current through the galvanometer
And Vg is the voltage through the galvanometer.
Wheatstone Bridge Errors
The Wheatstone Bridge is a conventional method of calculating resistance and can cause some inaccuracies. The following are the errors faced in the Wheatstone Bridge:
- There can be an error in the measurement due to a difference between the mark value and the true value of the three known resistances in the circuit.
- Furthermore, the resistance value of the bridge changes due to self-heating issues. This can also cause a flaw in the calculation.
- The galvanometer, which is used to measure the voltage, is less sensitive. So, an inaccuracy may occur while measuring the balance point.
- While calculating low-value resistance, the thermal emf (E) can make the equation faulty.
- There can be personal errors while measuring the reading on the galvanometer. This error occurs especially while trying to find the null point.
Conclusion
The Wheatstone Bridge is one of the simplest circuits in physics and contributes greatly to the construction of devices such as sensors and testers. It helps in detecting the resistance through a device; however, it comes with a few errors. Further, you can read about Wheatstone Bridge working principles to improve your knowledge on this topic.
