Page:Collected Physical Papers.djvu/255

Rh Table I.—Showing the Variation of Resistance with E. M. F.

It will be seen from the above table that as the E. M. F. increased from 0·05 volt to 0·45 volt the resistance decreased continuously from 2,500 to 640 ohms, i.e., about one-fourth its original value, and this diminution of resistance or increase of conductivity is not abrupt, but continuous.

In the lower portion of the curve, where the E. M. F. is low, the resistance is great and its variation small; but the curve soon becomes steep with the rise of the electromotive force. With a higher E. M. F. the change, to which the conductivity variation is due, proceeds very rapidly. This change is so great that at a certain critical value of E. M. F. it is almost abrupt. This is well seen in curve A, a little beyond 0·45 volt.

When the E. M. F. is adjusted to fall just short of the critical point, say, at 0·4 volt, then a slight increase of E. M. F., say +0·1 volt, will cause a very great variation of current; whereas if the E. M. F. had been so adjusted as to fall very much below the critical point, say, at 0·3 volt, an additional electromotive