A new form of magnetic balance has been devised for the accurate measurement of the intensity of field at any point in the air path of a magnetic circuit, the instrument being sufficiently delicate to detect differences in intensity of field amounting to one line per square centimetre, whereas existing methods measure only the average intensity over the whole field, or are only sensitive to differences of 150 to 200 lines/cm2.The distribution of the lines of force issuing from the sides of a bar magnet has been plotted and the surface of the magnet discovered not to be an equipotential surface.Biot-Savart's law has been experimentally proved.