Page:Collected Physical Papers.djvu/254

234. Just as the magnetic permeability of iron is not constant, but varies with the magnetic force acting on it, so the conductivity of metallic particles is not constant, but varies with the E. M. F. acting on them. The characteristic curves given above bear also a remarkable resemblance to the curves of magnetisation. Other parallelisms will be noted later. These similarities are probably due to the fact that in both cases we are dealing ultimately with phenomena of molecular deflection and rearrangement, taking place in one, under increasing magnetising force, and in the other, under increasing electromotive force.

As the absolute values of the current and the E. M. F. are known, it is easy to find the resistance of the receiver for any given E. M. F. The different values of the resistance can also be determined by the method of substitution. After the curve is drawn, leaving the paper still on the platform, a resistance box is substituted for the receiver, and the whole operation is repeated, with this difference, that now we have to change the resistance continuously in order to keep the galvanometer spot on the curve. The values of these resistances at definite points of the curve now correspond to the resistances of the receiver at those points. We can thus find the value of the resistance of the receiver at any point in the curve. This way of finding the resistance was used to supplement that of direct calculation $$\left ( R=\frac{E}{C} \right ) $$, from the known values of the E. M. F. and the corresponding current; the two results were found practically identical.

I give below a table showing the variation of resistance with E. M. F. for the curve A (fig. 52).