The weigth of a body decrease in a coal mine why?

Answer

Gravitational force is between two object caused by energy in mass. Here when you are in a mine the radius of the earth in case of calculating gravity is reduced to the length between you and the center.

This means the mass of earth is decreasing even though the distance is decreasing. Relative to the decrease in distance squared the decrease in the mass of the second object(earth) is more. As a result your weight decreases.

r=rho l/a where a is the area, l is the length and rho is resistivity so the greater the area the lesser the resistance thats why the earth pin is thicker bcos current follow the shortest path, so less resistive .