Three materials $A$, $B$ and $C$ have electrical conductivities $\sigma$, $2\sigma$ and $2\sigma$ respectively. Their number densities of free electrons are $n$, $2n$ and $n$ respectively. For which material, is the average collision time of free electrons maximum?

Two identical storage batteries, each having emf e and internal resistance $r$, are connected, as shown in Fig. Determine the potential difference set up between the points $A$ and $B$.

The variation of potential difference $V$ with length $l$ in case of two potentiometers $A$ and $B$ is as shown. Which of these two will you prefer for finding the emf of a cell or for comparing emfs of two primary cells? Or, which of the two potentiometers, $A$ and $B$, is more sensitive?

Answer the following question:

What is the order of magnitude of the resistance of a (dry) human body?