We have next to notice three important facts in electrostatics and some consequences flowing therefrom.
Electrostatics and Magnetism, p. 236.
Assoc. Report for 1867, or Lord Kelvin's Reprint of Papers on Electrostatics and Magnetism, p. 260.
In the next place we must notice that electrification is a measurable magnitude and in electrostatics is estimated in terms of a unit called the electrostatic unit of electric quantity.
The fact that there is no electric force in the interior of such a closed electrified shell is one of the most certainly ascertained facts in the science of electrostatics, and it enables us to demonstrate at once that particles of electricity attract and repel each other with a force which is inversely as the square of their distance.