Length of potentiometer wire is 5 m. It is connected with a battery of fixed e.m.f Null point is obtained for Daniel cell at 100 cm on it. If the length of the wire is made 7 m, then what will be the position of null point ?

Explanation

Let the e.m.f of battery be E volt, the potential gradient is
`K_1 = E/5 V//m`
When the length of poteniometer wire is 7 m potential gradient is `k_2 = E/7 V//m`
Now, if null point is obtained at length `l_2` , then `E_2 1= k_2 l_2 = E/7 l_2 V`
Here same cell is balanced in two arrangements , hence
`E/5 l_1 = E/7 l_2 " "l_1 = 1 m`
`" "l_2 = 7//5 = 1.4 m`