When the river and seawater are in contact with the ghost, the Electric Potential Difference is created by the movement of $Na+$ ion and $CL-$ion. The transport number of $NA+$ion is $0\mathrm{.}39.$

$($a$)$ Which electrical potential is bigger in the area where the river and the sea meet?

$($b$)$ Check the result of $($A$)$ by inducing the equation to find an electric potential differentce using electrochemical equilibrium conditions.