To measure the low resistance by the Kelvin’s double bridge method.
OBJECT: To measure the low resistance by the Kelvin’s double bridge method.
APPARATUS REQUIRED:
Bridge set, rheostat, galvanometer, connecting leads, millimeter, and D.C. source.
THEORY:
The Kelvin’s bridge is a modification of Wheatstone stone bridge and provides greatly increased accuracy in the measurement of low value resistance. An understanding of the Kelvin’s bridge arrangement may be obtain by study of the difficulties that arrives in the whetstone bridge on account of the resistance of the leads and the correct while measuring low value resistors.
We consider the bridge “r” represent the “R” to standard resistance “S” to galvanometer connected indicated by doted line. The connection will be either indicated by dots to point “m” or to point “n”. When G is connected to the point in the resistance “r” of the connecting leads is added to the standard resistance “S” resulting in the indication of to low and indication for unknown resistance “R”. When the connection is made the point “n” the resistance “r” is added to the unknown resistance resulting in the indication of too high value for “R”.
We assume that
P/Q=p/q
E_{ac} = iR+i(p+q)r\(p+q+r)+ i_{s}
E_{ab} = p*E_{ac}/P+Q
E_{ad} = iR+ir*p\p+qr
E_{ab} = E_{ad} (at the balance condition)
P\P+Q E_{ac }= i(R+rp/r+p+q)
Substitute the value of E_{ac} then
R = P\Q*S+qr\p+q+r[P/Qp/q]
R =P/Q*S
It indicates that the r has no effect on the measurement.
CIRCUIT DIAGRAM:
Fig. 7.1 Kelvin’s double bridge
OBSERVATION TABLE:
S. No. 
Sr 
Normal 
R =P\Q*S 
Reverse 
P\Q 
R =P\Q*S 



Ss(M ohm) 
P\Q 

So 
Ss 






RESULT: The value of low resistance is…………………..
comments (0)