Page:Collected Physical Papers.djvu/212



In the previous paper various effects have been described caused by visible and electric radiation. Considerations were adduced which tended to show that these effects were due to molecular strain induced in the substance by the action of radiation. The whole history of the change produced by radiation, both the direct and after-effects, were graphically recorded in the various curves. The strain effect produced in a substance was shown to be attended with conductivity or electromotive variation. We shall next inquire whether strain, which is undoubtedly produced by mechanical means, gives rise to response by conductivity and electromotive variations.

As regards the conductivity variation due to mechanical strain, it is well known that in the construction of standard resistance coils, winding the wire on a spool causes a distinct variation of resistance, and that this strain effect can only be removed by annealing. The difference between the resistance of a substance when strained and after it is annealed is sometimes very considerable.

The effect of electric radiation in changing the conductivity of a mass of discontinuous particles is very great. It is to be borne in mind that the effect of electric radiation is only skin-deep. As the action is one of surface, the larger the surface the greater is the effect produced. As already stated, the effective surface