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292 after its cessation, when a persisting electrical effect diminishing rapidly with time will be observed. When the wire is brought to the normal condition, successive responses to uniform stimuli are exactly similar in the case of metals which, like tin, show no fatigue. I usually interpose a high external resistance, varying from 1 to 5 megohms, so that the galvanometer deflections are proportional to the electromotive variations; the internal resistance of the cell and the variation of that resistance by the addition of chemical reagents are thus rendered quite negligible. Ordinarily tap-water is used as the electrolyte. The responses obtained with tap-water are practically the same as those obtained with distilled water. Zinc wires in ZnSO$4$ solution give responses similar in character to those given by, for example, Pt or Sn in water.

The following experiments show how the phenomenon of response is intimately connected with the molecular condition of the acted wire:—

Effect of Annealing.—The photographic records, given in fig. 76, show the equal and opposite responses