and in a prescribed position, deflecting the suspended bar from the magnetic meridian.
11. The torsion-bar and deflecting bar.—That the thread to which the magnet bar is suspended is without torsion in the mean position of the latter, is recognised thus: a brass bar of equal length and breadth, and of nearly equal weight, as the suspended magnet bar, having a small magnet inserted in it (in order somewhat to shorten the duration of the vibration due to the elasticity of the thread) is placed in the stirrup instead of the magnet bar. If the thread is without torsion, the magnetic axis of the small magnet will be in the same line as that of the larger bar was. In order to test this accurately, the auxiliary bar must, like the principal bar, be provided with a mirror and a mirror-holder. For measurements of intensity a second magnet bar of like dimensions to the principal bar is required, which may also be placed in the stirrup instead of the latter, in order to observe its vibrations, and to measure its moment of inertia. The same bar, however, must also serve as a deflecting bar, and for this purpose it is fitted into a small wooden case, which is bounded exteriorly by even surfaces and straight edges parallel to its magnetic axis, in order to give it its place quickly and accurately on the measuring scales.
12. The weights and weight-holders.—For measurements of intensity it is requisite that the deflecting bar may also be vibrated, and its moment of inertia thus deduced. For this purpose a thin wooden rod is placed across the vibrating magnet bar, and two equal weights are suspended, at various distances from each other, successively on both sides of the magnet bar. In order to mark the points of suspension, and to determine accurately their mutual distances, both weights, each of which amounts to 500 grammes, are provided with a small capsule. The capsule is placed on a fine point, projecting from the wooden rod. There must be several such projecting points at 50 millimetres distance from one another, with the exception of the two central ones, which are situated at 100 millimetres from each other. These distances must be measured with microscopical accuracy.
13. The quieting bar.—In order to perform the observations promptly and accurately, it is of importance to be able to moderate at pleasure the vibrations of the magnet bar; for instance,
