Page:On Polarisation of Electric Rays by Double Refracting Crystals.djvu/2

292 short wave lengths, I have succeeded in obtaining very satisfactory results with crystals of moderate size. These experiments show that certain crystals are double refracting as regards electric rays, and that they polarise the transmitted beam. With the help of a rudely constructed apparatus, I was able last year to detect traces of these effects. The apparatus has since been improved in detail; it is now possible to detect the polarisation effects with certainty.

The usual optical method of detecting the bi-refringent action of crystals, is to interpose the double refracting structure between two crossed Nicols. The interposition of the crystal generally brightens the dark field. This is known as the depolarising effect, and is regarded as a delicate test for double refracting substances. There is however, no depolarising effect, when the principal plane of the crystal coincides With the polarisation planes of either the polariser or analyser. The field also remains dark, when the optical axis of the crystal is parallel to the incident ray.

A similar method was adopted for experimenting with polarised electric radiation. The electric ray is first polarised by a wire grating. A similar grating acts as an analyser. The two gratings are crossed, and the crystal to be examined is interposed. The Receiver is a modified form of 'Coherer' with its associated Voltaic cell and Galvanometer. Brightening of the field is indicated by a throw of the Galvanometer needle.

Radiator.—A small Ruhmkorff's coil is used for the production of oscillatory discharges between two small metallic spheres, the diameter of each sphere being 1·5 c. m. The choice of a coil to produce electric oscillation has been a matter of necessity. I obtained oscillatory effects with ease and certainty by using a small inﬂuence machine of the Replenisher type. But in the damp atmosphere of Calcutta, the satisfactory working of such a machine is a matter of great difficulty, at least for the greater portion of the year. I had therefore to abandon the influence machine with regret, and to use a Ruhmkorff's coil instead. This coil caused me the greatest trouble. The discharge would of a sudden cease to be oscillatory; after a great deal of coaxing it would work satisfactorily just for a short time. The only coil I could get, was a badly constructed one, with defective insulation. I made it serviceable by changing the condenser and improving the vibrator. By looking to many points of detail I succeeded in making the apparatus work with fair uniformity for several hours. It must be borne in mind that the Receiving apparatus also requires careful adjustment.