Page:Collected Physical Papers.djvu/247

Rh curves of the three classes of substances thus belong to three distinct types, so that it is possible to determine the class to which the sensitive substance belongs by its characteristic curve.

These curves also throw much light on the obscure subject of the action of radiation on various sensitive substances. In the present Paper I shall describe in some detail the reaction of the first class of substances, i.e., those which show a diminution of resistance, and amongst these I shall take typical instances of non-recovering and self-recovering substances, and demonstrate the peculiarities of their cyclic characteristic curves. A brief reference will also be made of the characteristics of the second class which exhibit an increase of resistance.

Experimental Arrangements

The electric circuit consists of (l) the sensitive substance, (2) a sensitive dead-beat D'Arsonval galvanometer, and (3) a potentiometer arrangement for a gradual and continuous increase or diminution of E. M. F.

1. The Sensitive Substance.—Experiments were made with both single and multiple contacts. In the former case the pressure of contact was adjusted by means of a fine micrometer screw, or by means of springs. In some cases the contact ends were both rounded; in others, a pointed end pressed against a flat piece.

In the case of multiple contacts, a small quantity of metallic filings was put in a glass tube, and the fragments compressed between two electrodes, the pressure being regulated by a perfectly easy-running micrometer screw. The pressure was adjusted till a feeble initial current flowed through the circuit. The