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252 7. The curve obtained with strong, is steeper than that with feeble, initial current.

8. There is found, especially when the initial current is feeble, a critical E. M. F., at which the conductivity change becomes so rapid as to produce an almost abrupt bend in the curve. Stronger initial current appears not only to lower the critical point, but also to mitigate the abruptness of this change.

9. The effect of E. M. F. in modifying the conductivity of the surface layer is well seen in self-recovering substances. There is a definite conductivity corresponding to a definite E. M. F. As the E. M. F. is increased, the sensitive molecular layer is strained, and a definite increase of conductivity produced. When the increased stress is removed, the corresponding strain also disappears, and there is an elastic recovery to its former molecular and conductive state. Hence, when it is carried through a complete cycle of electro-motive variation, with moderate speed, the forward and return curves coincide, and the substance regains, at the end of the cycle, its original molecular condition.

10. This is the case where there is complete recovery on the removal of the stress. With non-recovering substances we find an outstanding residual effect. In a curve taken with cyclic electromotive variation, the forward and return curves do not coincide, but enclose an area. There is a hysteresis. The larger the range of the electromotive variation the greater is the area enclosed. There is a residual conductivity variation, at the end of the cycle, which may be dissipated by mechanical vibration.

(British Association at Glasgow, Section A, September 1901.)