New magnetic intensity‐variometers

1. Two like declination needles, one lying above the other, with a common axis of rotation, are brought into such a position that they cross at right angles to each other, each making an angle of 45° with the magnetic meridian; then, on turning the magnets, each of the four poles moves in an almost homogeneous magnetic field, as is apparent from the known distribution of the lines of force of bar magnets, and a change in the exterior directive force, the horizontal intensity of the earth's magnetism, corresponds to a proportional turning of the needles. This is the fundamental principle of the instruments now to be described. 2. Description of a simple local‐variometer for the horizontal intensity . Two rhombic magnetic needles of tungsten steel, 5.7 cm. in length and about 1.4 g. in weight, which, treated according to Strouhal and Barus, possess a magnetic moment of about 30 C. G. S., oscillate on pivots, above dampers of copper plate, in a cylindrical case, with a vertical distance between them of about 6.4 cm., which, by raising and lowering the lower pivot with a screw and a check‐nut, can be varied and can be precisely fixed, The lower needle carries two light aluminum indices, perpendicular to its axis and pointing upward, which move, at the ends of the upper needle, over small graduations on silvered glass attached to it. The turning of the needles relative to one another is determined at different points, and the difference of the readings on each scale for the two positions of equilibrium of the needles, 90° apart, is found. The height of the whole instrument, which rests on three foot screws, is 16 cm., the diameter of the base 15 cm., its weight, 1.5 kg,; it is, therefore, quite portable.