The length of a wire of a potentiometer is $100 \, cm$,and the $emf$ of its standard cell is $E \, volt$. It is employed to measure the $emf$ of a battery whose internal resistance is $0.5 \, \Omega$. If the balance point is obtained at $l = 30 \, cm$ from the positive end,the $emf$ of the battery is (where $i$ is the current in the potentiometer wire).
- A$\frac{30E}{100}$
- B$\frac{30E}{100.5}$
- C$\frac{30E}{100 - 0.5}$
- D$\frac{30(E - 0.5i)}{100}$
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