Page:Electricity (1912) Kapp.djvu/94

90 charged sphere. We might represent each line by a straight wire stuck into the surface of the sphere and pointing true to the centre. We should thus get a kind of spherical hedgehog; to represent a strong field due to a large charge we should stick into the sphere more wires, and to represent a weak charge we should use a smaller number of wires, but in all cases the wires would be evenly distributed all over the sphere. An imaginary sphere, laid round the nucleus from which all the wires spring, will be pierced by all the wires whatever may be the radius of this imaginary sphere, but the number of wires piercing a unit of the surface of the imaginary sphere will be inversely proportional to the square of its radius. But we know that the force is also proportional to the inverse square of the distance from the centre. The two things follow the same law, and it is therefore obvious that by a suitable selection of units we may express the force at any point by a number indicating how many wires pierce a unit of the surface of the imaginary sphere laid through the point. Thus the density of the lines of force passing through the surface at the point in question is a measure of the mechanical force exerted on unit charge at