that point. This is the dynamic significance of the conception of lines of force introduced by Faraday. The density, or number of lines to the unit surface is called the electric "induction," and the force experienced by unit charge is, then, simply equal to the induction, whilst the mechanical force experienced by a body charged with q units will be q times as great. Writing the symbol B for the induction, the force is given by the formula
B may be considered as of the nature of a flow of force, or "flux," piercing each square centimetre of the imaginary sphere laid through the point in question, and the total flux emanating from the charged nucleus will then be represented by the product of B and the total surface of the imaginary sphere laid round it. It will also in our mechanical model be represented by the total number of wires we have stuck into this nucleus. But the total number is the same whatever be the radius of the imaginary sphere. To get the relation between original charge Q on the nucleus and total flow of force emanating from it, we may therefore choose any radius.
