The emf of the driver cell of a potentiometer is 2V & its internal resistance is negligible . The length of the potentiometer wire is 100cm & resistance is 5 ohm.How much resistance is to be connected in series with the potentiometer wire to have a potential gradient of 0.05V/cm?
 
Solution) Potential gradient K= 0.05 V/cm = 5V/m
Driver cell emf E=2V
Resistance of potentiometer wire R1= 5Ω/m
Series resistance, Rs= ?

Now, to have K=5V/m, current flowing through the wire must be 1 Ampere (  since current I= 5V/5Ω=1A)
So, for the driver cell of emf 2V to generate 1A of current, series resistance ro be connected is 2Ω( since Rs= E/I=2/1=2Ω)

Therefore, Rs=2Ω