Page:Collected Physical Papers.djvu/304

284 relative effects of a series of compounds like Na$2$CO$3$, K$2$CO$3$, etc. Balance having been previously obtained between the normal sensitiveness of A and B, the two different solutions are now applied at the two points; the slightest difference in their relative action is at once exhibited by the upsetting of the balance during stimulation, the direction of the resultant deflection indicating the more stimulating reagent.

From what has been said, it will be seen that by rendering A and B unequally excitable, a resultant response may be obtained. The block may be abolished, and the wire may be vibrated as a whole; the response will now be due to the differential effect at A and B. For producing difference in excitability we may subject one point, say A, to a preliminary vibration, or apply at the point a suitable chemical reagent. By the application of the latter there will be a small P. D. between A and B, which will simply produce a displacement of the zero. (By means of a potentiometer the galvanometer spot may be brought back to the original position). The displacement of the zero does not affect the general result. The direction of this more or less permanent current, due to the small P. D., gives no indication of the direction of current of response; the direction of the latter is determined by the rule that the responsive current flows towards the more excitable point. The electromotive response induced by mechanical stimulation is algebraically superposed on the existing P. D.