If σ1, σ2 and σ3 are conductances of three conductors then their equivalent conductance if they are joined in series will be
1
σ1 + σ2 + σ3
2
\(\frac{1}{\sigma_1}+\frac{1}{\sigma_2}+\frac{1}{\sigma_3}\)
3
\(\frac{\sigma_1 \sigma_2 \sigma_3}{\sigma_1+\sigma_2+\sigma_3}\)
4
None of these