Page:Collected Physical Papers.djvu/256

236 force of the same amount 0·1 volt, produces a relatively small variation of current. In the former case, conditions stood as it were on the brink of a precipice, and a slight additional impetus precipitated a fall. It will thus be seen that if the E. M. F. acting on the receiver be so adjusted as to be near the critical point, a slight electromotive variation will produce a great change, exhibited by a very considerable increase of conductivity. It is interesting to note in this connection that I found the sensitiveness of receivers to electric radiation could be greatly enhanced by raising the E. M. F. acting on the circuit to a point as high as they could bear, just short of electric instability.

Influence of Intensity of Initial Current in Modifying the Characteristic Curve

When at the beginning of an experiment the receiver is adjusted at a definite low E. M. F., say, 0·05 volt, we can have any initial current, according to the adjusted pressure of contact, the current increasing with increase of pressure. We may thus start with a large or a small initial current, the E. M. F. at starting being constant.

In the curve B (fig. 52) the initial current is a little more than double that for A. It will be seen that the curve has become distinctly steeper; the critical point is at the same time lowered, from 0·46 volt as in the last case, to 0·43 volt. The sudden bend in the curve is now less pronounced. In connection with the lowering of the critical point by pressure, it is also interesting to note that by increasing the pressure within certain limits, I succeeded in greatly enhancing the sensitiveness of the receiver to the action of electric radiation.