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Rh to be presently described. By means of a variable diaphragm at the end of the tube, the amount of electric radiation for an individual experiment may be varied.

Next to the Polariser is the crystal-holder, which allows the principal plane of the crystal to be inclined at any azimuth.

The Analyser is similar in construction to the Polariser, and is mounted at the open end of the cell containing the sensitive Receiver, which is a spiral spring coherer, with numerous points of contact.

The resistance of the coherer varies within wide limits. The one I use gives the best result when the resistance is reduced by compression to about 4,000 ohms; the corresponding current circulating round the circuit is then about 10$-4$ ampere. The incident radiation reduces this resistance to 10 ohms or less. The sudden increase of current, due to the diminution of resistance, produces a deflection of the spot of light reflected from the galvanometer.

The coherer is enclosed in a small metallic cell open in front for the reception of the Analyser. The metallic cell has also a tubular projection behind, through which the wires from the ends of the coherer pass out.

Adjustable E. M. F. and the Galvanometer

The wires from the ends of the coherer lead to an adjustable E. M. F. and a dead-beat galvanometer of D'Arsonval type. The wires are placed within a double coating of tin-foil, and the galvanometer and the voltaic cell enclosed in a metallic case with a slit for the passage of the reflected spot of light. These precautions are taken for shielding the receiving circuit from the disturbing action of stray radiations.