measured with the accuracy it deserves only with the magnetometer. It is therefore unnecessary to state that such extreme accuracy cannot be attained with the small apparatus. And in order to obtain with it a good approximation, it must combine all the advantages of which it is susceptible.
The difficulty of an accurate measurement of intensity, with other instruments than the magnetometer, is thus stated in the memoir "On Terrestrial Magnetism and the Magnetometer:"
"In all cases, if the elimination is to be satisfactory, the experiments must not be performed at too small distances; consequently the effects are always comparatively small, and the means previously in use are inadequate to measure them with the necessary precision. It is this difficulty which has called for, and has given rise to the construction of a new apparatus, which may with propriety receive the name of magnetometer, since it serves to execute, with an accuracy equaling that of the most delicate astronomical determinations, all measurements—both of the force of magnetic needles, and of the intensity of the earth's magnetism (at least its horizontal portion). The (horizontal) direction of the earth's magnetic force is determined accurately with it to within one or two seconds of arc; the commencement and termination of a vibration is observed with it to within a few hundredths of a second of time, and consequently more accurately than the passage of stars behind the wires of a transit."
There are two circumstances, chiefly, on which the accuracy of an absolute measurement of intensity depends; first, the magnitude of the deflection produced; secondly, the delicacy of the instrument in measuring this deflection. In constructing an apparatus for this purpose we may therefore follow two different paths: we may either make the amount of deflection the main object, and pay only as much attention to the means of measurement as may be consistent therewith;—or we may attend chiefly to accuracy in the means of measurement, and let the amount of the deflection be the second object. The latter plan leads to much greater accuracy than the former, for this reason: the amount of deflection soon attains a limit, on account of the necessary condition of a considerable distance between the deflecting bar and the needle, so that the deflection produced
