MAGNETOMETER

An instrument for measuring the intensity of terrestrial magnetism. The three elements sought to be deduced from magnetic observations are, the declination, the inclination or dip, and the absolute intensity, together with the variations to which they are subject; and each of these elements requires for its determination a peculiar apparatus. A detailed account of the construction and various adjustments of the instruments made use of in the magnetic observatories recently established, and of the methods of observing, will be found in the Report of the President and Council of the Royal Society on the Objects of Scientific Inquiry in Physics and Meteorology, 1840. The first accurate measurements of magnetic force were made by Coulomb with the torsion balance; but the more precise method which is now generally adopted, and which depends on the dynamical principle of observing the number of vibrations made by a needle in a given time, was proposed by Graham so long ago as 1722. [s. 710]