Page:On Polarisation of Electric Rays by Double Refracting Crystals.djvu/3

1895.] Among the possible causes of unsteadiness may be mentioned the following—

lst. The current actuating the coil may vary after a time. To overcome this difficulty a fairly constant battery was made to charge a small storage cell, and a derived circuit from this cell was led to the Primary coil.

2nd. The interrupter may have its rate of vibration changed by heating, wearing out of contact points, and other causes. Any change in the periodicity of the vibrator is at once made evident by the corresponding change in the pitch of the note given out by the vibrator.

3rd. The sparking balls may have their surfaces roughened by the disintegrating action of the spark. To avoid this difficulty, the balls were thickly coated with deposit of gold, and were turned round at intervals to expose fresh surfaces.

The coil with a storage cell is enclosed, with the exception of a horizontal tubular opening, inside a metallic box, not dissimilar in appearance to an Optical Lantern. The interrupter is actuated by turning a key from outside. The sparking balls are at one end of a brass tube 25 c.m. long and 5 c.m. in diameter. At the further end of the tube is the Polariser. Inside the tube is placed a convex lens with the spark gap at its principal focus. With the help of the lens and suitable diaphragms, the electrical beam is made approximately parallel. By means of an Iris diaphragm, the amount of radiation may be varied.

Polariser.—The success of the experiment depends greatly on the care with which the Polariser and Analyser are constructed. Fine copper wire ·2 m. m. in diameter is carefully wound in parallel lines, round two thin sheets of mica. There are about 25 lines for every centimetre. The mica pieces are then immersed in melted paraffin, and the wires thus fixed in situ. By cutting round, two circular pieces, containing the gratings are obtained. The mica pieces are too thin to produce any disturbing effect. The gratings are fixed with wires parallel, at the ends of a tube 5 c.m. long. This Polariser tube rotates inside the outer end of the tube which sends out the parallel electric beam.

Analyser.—The Analyser is similar in construction to the Polariser. It rotates inside the Receiving tube, which contains the sensitive surface for detecting radiation.

Receiver.—The Receiving apparatus consists of a 'Coherer' with a Voltaic cell and Galvanometer in series. The Coherer is modiﬁed from its usual tubular form. The filings, a single layer thick, are spread over a large surface. This arrangement secures great sensitiveness. A pair of insulated wires from the ends of the Coherer, are led out to a distant dead-beat Galvanometer of D'Arsonval type in series with a constant