The reciprocal of resistance is

Two metal wires of the same length and same area of cross-section have conductivities of their material $\sigma_1$ and $\sigma_2$. If they are connected in series,the effective conductivity is:

$A$ wire of resistance $x \ \Omega$ is drawn out,so that its length is increased to twice its original length,and its new resistance becomes $20 \ \Omega$. Then $x$ will be ........ $\Omega$.

The conductivity of a conductor decreases with temperature because,on heating:

The following four wires are made of the same material and are at the same temperature. Which one of them has the highest electrical resistance?

The electrical resistance of an iron wire is $R$. If both its length and radius are doubled,then: