In an experiment on the measurement of internal resistance of as cell by using a potentionmeter, when the key `K` is kept open then balancing length is obtained at y metre. When the key `K` is closed and some resistance `R` is inserted in the resistance box, then the balancing length is found to be `x` metre. Then the internal resistance is

Options

A. `((x-y))/y R`
B. `((y-x))/xR`
C. `((y-x))/yR`
D. `((x-y))/xR`

Answer

B

Explanation

`r=R(l_1/l_2-1)`
`=R=(y/x=1)=((y-x)/x)R`