Current Electricity

Wheatstone Bridge, Meter Bridge and Potentiometer

Every other day, science presents us with one or more ways to feel amazed. There are a host of experiments that show both how we can use things and make newer things out of them. Experiments related to Wheatstone Bridge and the potentiometer are among few such things in science that invoke a curious sense of amazement. Let us study more about the concept of Wheatstone bridge and meter bridge, along with potentiometer.

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The Concept of Wheatstone Bridge

Defined simply, a Wheatstone Bridge is an electric circuit that is used to measure the electrical resistance of a circuit. The circuit is set out by balancing two legs of a bridge circuit. Out of the two, one of the legs is an unknown component which was invented by Samuel Hunter Christie in the year 1833 and later, it improved and popularized by Sir Charles Wheatstone in the year 1843.

Nowadays, technological progress has allowed us to make various measurements through sophisticated tools and machines. However, even today, the wheat bridge remains an authentic way to measure electric resistance, down to the closest milliohms as well.

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Wheatstone Bridge

The Principle behind the Wheatstone Bridge

The usual arrangement of the Wheat stone bridge circuit has four arms. The bridge circuit where the arms are situated consist of electrical resistance. Out of these resistances, P and Q are the fixed electrical resistances and these two arms are the ratio arms. Next, A Galvanometer connects between the terminals B and D through a switch K2. The source of voltage of this arrangement is connected to the terminals A and C through a switch, K1.

A variable resistor S is connected between point C and D. The potential at point D is altered by adjusting the value of a variable resistor. If a variation in the electrical resistance value of arm CD is brought, the value of current I2 will also vary as the voltage across both A and C is fixed.

If we continue to adjust the variable resistance, a situation may come when the voltage drops across the resistor S that is I2. Here, S becomes exactly equal to the voltage drop across resistor Q that is I1. Q. So, the potential at point B becomes equal to the potential at point D hence the potential difference between these two points is zero hence current through galvanometer is nil. The deflection in the galvanometer is nil when the switch K2 is closed.

Applying Kirchoff’Law, in this condition,

P/Q = R/S

How is the Meter Bridge experiment carried out using the Wheatstone Principle?

The meter bridge experiment uses the wheat bridge experiment to demonstrate the resistance of an unknown conductor or to make a comparison between two unknown resistors. Through the above-stated equation, one can easily decipher the specific resistance of a given material

Conclusions of the wheat stone bridge principle are:

According to the Wheatstone-bridge principle, the resistance of length AB/resistance of length BC = R / X

Let l be the length of wire between A and B and then (100 – l) is the length of wire between B and C. Here, P = ρl / A. Since the wire has a uniform cross-section and ρ is constant. Its resistance is proportional to the length. That is P ∝ l, and Q ∝ (100–l). So,

L / (100–l) = R / X

This is how to draw the values of X for different values of R and the mean value gives the value of unknown resistance X.

The Concept of Potentiometer

A potentiometer is an electric device which is used to regulate EMF and internal resistance of a given cell. This helps in providing a variable resistance and therefore, a variable potential difference arising between two points in an electric circuit. It is basically a three-terminal resistor device with an adjustable arm that increases or reduces the resistance in the loop.


Potentiometer (Source: Wikipedia)

Solved Examples for You

Question: Describe how a potentiometer works in an arrangement.

Answer: A potentiometer consists of a uniform wire AB of manganin or constantan that has a length of usually 10 m. it is kept stretched between copper stripes that are fixed on a wooden board by the side of a metre scale. The wire is then divided into ten segments each of 1 m length.

These segments join in series through metal strips between points A and B. A steady current is maintained in the wire AB by a constant source of EMF Eo, called driver cell, that connects between A and B through a rheostat. A jockey slides over the potentiometer wire which makes contact with the wire and cell.


Potentiometer (Source: Wikimedia)

Thus we can say that the potential difference across any portion of the potential of the potentiometer wire is directly proportional to the length of that portion provided the current is uniform.

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