Now in all these theories of mathematical physics, the same type of procedure is invariably followed. Experimenters establish certain definite facts and detect precise numerical relationships between magnitudes, for example, between the intensity of an electric current flowing along a wire and the intensity and orientation of the magnetic field surrounding the wire. The mathematical physicist then enters upon the scene, assigns certain letters of the alphabet to the physical entities involved (in the present case electric current designated by
and magnetic intensity designated by
) and by this means translates the numerical relationships discovered by the experimenter into mathematical form. He thus obtains a mathematical relationship or equation
which is assumed to constitute the mathematical image of the concrete physical phenomenon
. His task will now be to extract from his mathematical equation or equations