In order to determine the e.m.f. of a storage battery it was connected in series with a standard cell (both are adding)  in a certain circuit and a current I₁ was obtained. When polarity of the standard cell is reversed, a current I₂. was obtained in the same direction as that of I₁. What is the
e.m.f. ${\epsilon }_1$ of the storage battery? The e.m.f. of the standard cell is ${\epsilon }_2$

• A

  ${\mathrm{\epsilon }}_1=\frac{{\mathrm{I}}_1+{\mathrm{I}}_2}{{\mathrm{I}}_1-{\mathrm{I}}_2}{\mathrm{\epsilon }}_2$

• B

  ${\mathrm{\epsilon }}_1=\frac{{\mathrm{I}}_1+{\mathrm{I}}_2}{{\mathrm{I}}_2-{\mathrm{I}}_1}{\mathrm{\epsilon }}_2$

• C

  ${\mathrm{\epsilon }}_1=\frac{{\mathrm{I}}_1+{\mathrm{I}}_2}{{\mathrm{I}}_1+{\mathrm{I}}_2}{\mathrm{\epsilon }}_2$

• D

  ${\mathrm{\epsilon }}_1=\frac{{\mathrm{I}}_2-{\mathrm{I}}_1}{{\mathrm{I}}_1+{\mathrm{I}}_2}{\mathrm{\epsilon }}_2$