Page:Collected Physical Papers.djvu/44

24

The apparatus used consists of an electric radiator emitting short waves, a large graduated circle provided with a central circular platform on which the refracting substance is placed, and a receiver, which responds to the electric radiation.

The Radiator.—Electric oscillation is produced by sparking between two metallic beads and an interposed metallic sphere one cm. in diameter. By a single sudden break of the primary in a Ruhmkorff's coil, a flash of radiation is emitted. The spark gap is placed at one end of a brass tube 5 cm. in diameter. By a sliding arrangement, the length of the tube can be varied. The Ruhmkorff's coil is enclosed in a copper box.

The Circle.—The circle has a diameter of 45 cm., and is graduated into degrees, but one-fourth of a degree can be easily estimated. The circle, as a whole, may rotate round a vertical axis which passes through the centre of a massive stand. There is a raised circular platform at the centre of the circle on which the refracting substance is placed. This platform carrying an index, can be rotated independently of the large circle. When the platform index is clamped, the two circles rotate together.

The Refracting Substance.—For substances which can be cast, the molten mass is poured into a cylindrical mould with a thin partition in the middle. In this way two equal semi-cylinders are obtained at each casting. Substances like wood or stone are turned, and the cylinder sawn into two equal halves; In my experiments, different sized cylinders were used; I have successfully used small ones with a radius of 8 cm. only. But when the cost of the material is not prohibitive,