Two cells of EMF $\varepsilon_1$ and $\varepsilon_2$ ($\varepsilon_1 > \varepsilon_2$) connected so that **like terminals are together** form a combination whose effective EMF (for the potentiometer sum-and-difference method) equals:
A$\varepsilon_1 + \varepsilon_2$
BZero
C$\varepsilon_1 \varepsilon_2$
D$\varepsilon_1 - \varepsilon_2$
Answer & Solution
Correct answer: D. $\varepsilon_1 - \varepsilon_2$
Like terminals connected → EMFs oppose → net EMF = $\varepsilon_1 - \varepsilon_2$ (difference method). With opposite terminals joined, EMFs add up to $\varepsilon_1 + \varepsilon_2$ (sum method).
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