Page:Collected Physical Papers.djvu/252

232 Characteristic Curve of a Single Point Iron Receiver

In order to reduce the conditions of experiment to their simplest, I first studied the effect of E. M. variation on Contact at a single point. The change induced was thus confined to the molecular layers at the point of contact. This consisted of a sharp point of iron, pressing against a convex iron surface, the pressure being capable of very delicate adjustment by means of a micrometer. Five different experiments were carried out with the same receiver.

The initial adjustments were made with an E. M. F. of 0·05 volt. The only difference made in the several experiments is as regards the initial current, caused by change in the pressure of contact.

In curve A (fig. 52) the initial current was the lowest value of the series. The pressure of contact was adjusted till the initial current at 0·05 volt was 2/10$5$ ampere. The E. M. F. was now continuously increased by the turning of the winding wheel, and the curve obtained in the manner previously described.

It will be seen that the curve is not straight, but concave to the axis representing the current. As the E. M. F. is increased, the current increases at a greater ratio. I may say here, in anticipation, that this appears to be a characteristic of the positive class of substances, i.e., of those which, like iron, exhibit a diminution of resistance under electric radiation.

The resistance of the receiver is thus not constant, but undergoes a continuous decrease with increasing E. M. F. The conductivity is therefore increased with the rise of E. M. F. The curve becomes steeper as the E. M. F. is increased, the conductivity undergoing a rapid and continuous increase.