In this topic we will discuss about Wheatstone bridge ,its principle and balanced condition for Wheatstone bridge .And some important points for Wheatstone bridge.
Before to know about Wheatstone bridge , students must read these topics first-
Wheatstone bridge –
It is an arrangement in which four resistances are arranged in the form of a bridge which is used for measuring one unknown resistance in terms of other three resistances .
Wheatstone bridge principle – It’s principle states that when four resistances P,Q,R and S are connected with a galvanometer and a cell in the form of bridge as shown in figure , the bridge is said to be balanced when there is no deflection shown by the galvanometer for this condition P/Q = R/S .
As shown in figure , resistance P is in arm AB , Q is in arm BC , R is in arm AD and S is in arm DC . A cell of emf ‘ε’ and galvanometer is connected as shown in figure with keys K1 and K2 . On closing key K1 and then key K2 if galvanometer shows no deflection then it is said that bridge is balance , and we will get P/Q = R/S .
As shown in figure I is the current flowing through the cell , when it reaches to point A it divided in two parts I1 towards AB and ( I-I1 ) towards AD . Again when I1 reaches to B then current Ig flows through galvanometer and ( I1-Ig) towards C and current I-I1+Ig towards DC .
Now applying Kirchhoff’s loop law in loop ABDA we get,
Again from loop BCDB we get (I1-Ig)Q – ( I-I1+Ig)S-IgG=0………………(II)
If galvanometer shows no deflection i.e. Ig=0 then eq (ii) may be written as
I1P = (I-I1) R ………………….(III)
And eq. (ii) may be written as ,
I1Q = (I-I1) S ……………………..(IV)
Dividing (iii) by (iv) we get P/Q =R/S This proves wheat-stone bridge .
Two view the video on Wheatstone bridge click here-
*****Important point for wheat-stone bridge ***
(1) Wheat-stone bridge is a null method , hence the measurement of unknown resistance is very accurate.
(2) Wheat-stone bridge remain unaffected by the internal resistance of the cell and galvanometer.
(3) it is not used to measure very high or very low resistances .
(4) It becomes very sensitive when resistance of all four arms are nearly same .