those days knew something of electricity, and it knew something of magnetism, but it knew these two things as distinct from each other. Now by one stroke of experimental genius Oersted showed the scientific world that, after all, electricity and magnetism are not independent domains of physics, but are intimately connected. Every physicist in Europe repeated the experiment, and many speculated on the question what really constituted the connecting link between magnetic and electric phenomena. It was Ampere who first formulated a rule by which the direction of deflection could be predicted, and two other French scientists, Messrs. Biot and Savart, gave a mathematical formula by which the magnitude of the deflecting force could be calculated. Ampere's rule is as follows: Imagine yourself swimming in the direction of the electric current and looking at the compass needle. Its north end will then be deflected to your left. Biot-Savart's law may be stated by reference to the force exerted by the current on unit magnetic matter at any given point of the space in the neighbourhood of the conductor.
Before discussing this subject it is necessary to define what we mean by the term "unit
