Two metal wires of identical dimensions are connected in series. If $\sigma $1 and $\sigma $2 are the conductivity of the combination is
${{{\sigma _1} + {\sigma _2}} \over {{\sigma _1}{\sigma _2}}}$
${{{\sigma _1}{\sigma _2}} \over {{\sigma _1} + {\sigma _2}}}$
${{2{\sigma _1}{\sigma _2}} \over {{\sigma _1} + {\sigma _2}}}$
${{{\sigma _1} + {\sigma _2}} \over {2{\sigma _1}{\sigma _2}}}$
