3. The Damping Arrangement.-The arrangement for securing aperiodicity is a form of liquid damping, with a fine adjustment for regulating the amount. The staff carrying the magnets is prolonged downwards past the lower coil, and ends in a vane of aluminium. This vane may be spear-shaped or "paraboloid" in section. This vane hangs on a cup of oil or other fluid, d, which cup can be raised or lowered by means of a fine micrometer screw (50 turns to the inch), moved by a large milled-head, e. By this means the vane can be made just to touch the "skin" of the liquid, or can be partly or wholly submerged, and in this way any degree of aperiodicity can be secured.

This arrangement has a special value in obviating a difficulty in many laboratories where the support for the galvanometer is attached to the wall of the building, and where the instrument is accordingly affected by movements in the same or other rooms. These tremulous oscillations are very troublesome, but they can be obviated by having the vane sharply pointed and the cup so adjusted that the tip just touches the "skin" of the oil. The magnetic system thus works as it were between two frictionless (or almost frictionless) pivots-the fibre above and the point of the vane on the liquid below; and, unless the vibration in the instrument is very great indeed, it does not appear in the index.

For perfect aperiodicity, without loss of sensitivity, I find the blunt vane preferable.

The framing and case of the instrument were made by Mr William Hume, Edinburgh. The magnetic system was made by Messrs Elliott Brothers, London, and the coils were wound by Messrs Nalder Brothers, London.

The total resistance of the four coils in series is 12,650 ohms.