Page:Collected Physical Papers.djvu/269

between the contact surfaces, for the process described above is anything but abrupt. Again, fusion would have produced a permanent conductivity change, but in the case under review the change is not permanent. Thus, for example, when the applied E. M. F. was 0·2 volt, the current was represented by a deflection of two divisions, the resistance being 50,000 ohms. When the E. M. F. was raised to 0·7 volt, the current increased to 40 divisions, indicating a diminution of resistance which remained at the definite value of 8,750 ohms. If the E. M. F. was again reduced to its original value of 0·2 volt the current was once more two divisions, and the resistance rose to exactly its original value of 50,000 ohms. It would thus appear that—

(1) For a particular E. M. F. there is a definite value of conductivity.

(2) When in a given part of the curve the E. M. F. is increased by a definite amount, the conductivity is also increased in a definite manner. The increased stress produces a definite conductivity distortion, and on the removal of the stress there is a quasi-elastic recovery of its original conductivity.

(3) As the seat of these changes is in the molecular layers at the definite single-point contact, it would appear that the conductivity variation and its recovery are due to molecular distortion and subsequent elastic recovery.

What has been said above of the conductivity change under electromotive variation and the complete recovery is true only when the cycle is completed at a moderate speed, during which, time is allowed for the completion of, or the recovery from, the induced