Wheatstone’s bridge

The Wheatstone bridge was designed by Charles Wheatstone in order to measure the unknown values of resistance and is also used as a means of calibrating measuring instruments like the voltmeters, ammeters, etc, with the help of a long slide wire. The Wheatstone bridge can be used for measuring extremely low values of resistances in the mili-Ohms range.

What exactly is the Wheatstone bridge?

A Wheatstone bridge is basically an electrical circuit used for the measurement of an unfamiliar electrical resistance by balancing the two legs of a bridge circuit, where one of its legs includes the undetermined component.   

The Wheatstone bridge is used for the accurate measurement of resistance. The bridge contains two known resistors, one variable and one undetermined resistor which are connected in the form a bridge as shown in the figure below. The variable resistor is adjusted in such a way that the current through the galvanometer is reduced to zero. Hence, when the current through the galvanometer is reduced to zero, the ratio of the two known resistors is precisely the same as the ratio of the variable resistance and the unknown resistance. Hence, in this way it becomes quite simple to calculate the value of the unknown resistance with the help of Wheatstone bridge.

Wheatstone bridge

The Wheatstone bridge circuit is in the form of two simple series which are a sort of parallel arrangements of resistors connected between a terminal of voltage supply and the ground which produces zero voltage difference when the parallel resistor legs are balanced. The

Wheatstone bridge circuit is in the form of a diamond and has two input terminals and two output terminals. It consists of four resistors arranged in the from of a diamond.     

For a certain adjustment of Q, VBD = 0, therefore no current flows through the galvanometer.

VB = VD or VAB = VAD  

Or l1.P = l2.R 

Wheatstone bridge

Likewise, VBC = VDC Þ l1.Q = l2.S  

Dividing (1) by (2), we get, P / Q = R / S  

The following video will further improve your knowledge on Wheatstone bridge 

Now we discuss some of the solved problems based on Wheatstone bridge:

Illustration:  

What’s the effective resistance of the circuits shown in the adjacent figure?

effective resistance of the circuits shown

Solution:  

(a)  It is a Wheatstone bridge that is balanced. Hence, the central resistance labeled ‘C’ can be assumed as ineffective.

        Req = R.

(b) The resistance R is in parallel with a balanced Wheatstone bridge.

        Req = R.R / R + R = R / 2  


Wheatstone bridge is of immense importance and can be used in a number of applications.  

askIITians offers comprehensive study material which covers all the important topics of Physics of IIT JEE. Various topics like effective resistance of Wheatstone bridge, and various solved problems on Wheatstone bridge have also been included. It is important to master this topic in order to remain competitive in the JEE.

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