Two metallic wires of identical dimensions are connected in series. If \( \sigma_1 \) and \( \sigma_2 \) are the conductivities of the these wires respectively, the effective conductivity of the combination is:
\( \dfrac{\sigma_1 \sigma_2}{\sigma_1 + \sigma_2} \)
\( \dfrac{2\sigma_1 \sigma_2}{\sigma_1 + \sigma_2} \)
\( \dfrac{\sigma_1 + \sigma_2}{2\sigma_1 \sigma_2} \)
\( \dfrac{\sigma_1 + \sigma_2}{\sigma_1 \sigma_2} \)