Page:Scientific Memoirs, Vol. 2 (1841).djvu/84

72 needle is thereby removed. The measurement is still dependent on the remaining peculiarity of the needle, namely, its own magnetism; and as soon as we can reduce this to an absolute measure, the force of terrestrial magnetism itself may also be reduced to an absolute measure; for we have only to divide the number which expresses the moment of rotation by the number which measures the magnetism of the needle. In fact, the earth's magnetism is measured by a force equal to itself, whose action on the unit of magnetism of the needle consists in a moment of rotation, measured by the force which the unit of weight exerts on a lever of the unit of length."

If, therefore, $$T$$ signifies the terrestrial magnetism, and $$M$$ the magnetism of the needle, or of the vibrating bar,

"We might be inclined to suppose that the weight which a magnetic needle can carry would afford a standard by which the force of magnetism developed in the needle might be estimated; but a closer examination will show that this method is quite unavailing for our purpose. The determination itself is incapable of much precision; for repeated experiments give very different results; but there is a still more important objection: the capability of sustaining weight has no necessary connexion with the magnitude of the development of magnetism in the needle, in the sense in which it must here be understood. The moment of rotation is due to the magnetism of all the parts of the needle, upon which the terrestrial magnetism acts equally, and in parallel directions. The sustaining power, on the contrary, is chiefly due to the magnetism situated in the ends nearest to the weight, which, moreover, is modified every moment by the reciprocal action of the magnet-bar and the suspended iron.

"A magnetic needle, at a given place, acts on, every point of space, in an amount and direction determined by its distance and position. In the immediate neighbourhood its action is strong, but very unequal on different parts; at great distances the action is weak, but almost uniform in strength and direction within a moderate space. The greater the distance, the nearer the law of the force approaches to a rule, which is very simple, and is completely given by theory: we may limit ourselves here to the consideration of a single case, which is sufficient for our purpose.