Page:Elementary Text-book of Physics (Anthony, 1897).djvu/355



289. The Magnetic Field of a Current.— Soon after the discovery by Oersted of the force exerted by an electrical current on a magnet, Biot and Savart instituted experiments to discover the law of this force. They suspended a small magnet near a long vertical wire through which a current was passing and counteracted the earth's magnetic field by magnets, so that, if no current passed through the wire, the small magnet was free from any directive force. When the current passed, the magnet placed itself at right angles to the plane containing the wire and the centre of the magnet. By oscillating the magnet, the strength of the magnetic field acting upon the magnet was found to be directly proportional to the strength of the current and inversely proportional to the distance between the magnet and the current.

It follows at once, from the position assumed by the short magnet, that the lines of magnetic force set up by the current are circles with their centres in the current. The relation between the direction of the current and the direction of the lines of force set up by it may be described in several ways. Ampère's rule is as follows: If the observer imagine himself swimming with the current and looking toward the magnet, the north pole of the magnet tends to move toward his left. Maxwell's rule is, that the direction of the current and the direction of its lines of force are related as the directions of translation and rotation of a right-handed screw. This rule is the one now commonly used. By supposing the wire carrying the current bent around into a closed curve, it will easily