Page:On a Complete Apparatus for the Study of the Properties of Electric Waves.djvu/13

Rh The receiver carrying the analyser is also capable of rotation round a horizontal axis by means of a tangent screw. The angular rotation is measured by means of an index fixed to the analyser, and a graduated vertical disk.

The gratings are made by winding fine copper wire, parallel, round square frames, as used by both Hertz and Lodge. A series of parallel slits cut in a metallic plate serves the purpose very well. Other forms used—the serpentine and the jute polarizers—will be described later on.

The spark-gap is placed vertical, and the polarizer is adjusted with wires horizontal. The emergent beam is now completely polarized, the vibration taking place in a vertical plane passing through the axis.

The analyser fitted on to the receiver may be placed in two positions:—


 * (1) Parallel position. When both the gratings are horizontal.
 * (2) Crossed position. When the polarizing grating is horizontal, and the analysing grating vertical.

In the first position the radiation, being transmitted through both the gratings, falls on the sensitive surface, and the galvanometer responds. The field is then said to be bright. In the second position the radiation is extinguished by the crossed gratings, the galvanometer remains unaffected, and the field is said to be dark. But on interposing certain crystals with their principal planes inclined at 45° to the horizon, the field is partially restored, and the galvanometer spot sweeps across the scale. This is the so-called depolarization action of double-refracting substances.

Experiments with Wire Gratings.—A Wire grating at 45° interposed between the crossed analyser and polarizer partially restores the field, but ordinary wire gauze does not transmit any radiation, the action of one set of wires being neutralized by that of the other set at right angles.

Double Refraction Produced by Crystals.—The crystals to be examined are mounted on the holder, and properly inclined. Double refraction is shown by all crystals belonging to the Rhombic, Rhombohedral, Triclinic, and Monoclinic systems. The effects exhibited by the following are very marked, small pieces even producing depolarization.