A current of 1.0 mA  is flowing through a potentiometer wire of length 4 m and of  resistance 4Ω , the potential gradient of potentiometer wire is

Current i=1 mA=1×10−3A length l=4 m resistance R=4 Ω Potential gradient =Vl=IRl                              = (10−3)×44                              = 10−3 Vm−1