Page:Collected Physical Papers.djvu/260

240 portions of the curve represent the creeping effect during pause. The conductivity variation thus lags slightly behind the impressed electromotive variation. Certain effects due to this will be noticed later.

Characteristic Curves given by a Mass of Iron Filings

Owing to the multiplicity of contacts the conditions here are not so simple as in the case described above. It will, however, be seen from the curves given in figure 54, that the results are of the same general nature. The resistance undergoes diminution with increasing E. M. F. The curves are steeper with stronger initial currents. Greater intensity of initial current appears also to have the effect of lowering the critical point. I obtained similar results with Mg, and Ni.

It is difficult in one curve to represent adequately the variation of conductivity caused by small, moderate, and excessive increase of E. M. F. Broadly speaking, the curve may be divided into three parts. In the first, when the E. M. F. is low, the change is slight. The curve then becomes very steep in the second part, the conductivity variation being rapid. The increase of conductivity, however, reaches a limit, after which there is little further change. The whole curve is thus somewhat S-shaped.

Conductivity Variation with Cyclic Variation of E. M. F.

When conducting particles of the non-recovering, positive, or, iron type are subjected to sudden electric variatian due to radiation, a residual effect is observed