Page:Collected Physical Papers.djvu/298

278 the electric variations induced at A and B. The electric changes at A and B will continuously balance each other, and the resultant effect on the galvanometer will be zero, when the mechanical disturbance reaches A and B at the same time and with the same intensity, when the molecular condition is similar at the two points, and when the rate of rise and subsidence of disturbance is the same at the two points. In order that a resultant response may be exhibited, matters have to be so arranged that (1) the disturbance reaches one point, say A and not B, and vice versâ. This may be accomplished by the method of block. Again, a resultant differential action may be obtained even when the disturbance reaches both A and B, if the electrical excitability of one point is relatively exalted or depressed by physical or chemical means. Besides the method of the block there are thus two other means of obtaining a resultant response, (2) by the method of relative exaltation, (3) by the method of relative depression.

The electromotive effect described below can be obtained with all metals. A piece of "tin" wire (an alloy of tin and lead used as electric fuse) will be found to give very good results. A specimen of wire 1 mm. in diameter, 10 cm. in length, is mounted in the apparatus. Two strips of cloth moistened with water or dilute salt solution are securely tied round two points A and B. Electric connections are made with the indicating galvanometer by means of non-polarisable electrodes (Zn in ZnSO4 solution). Special precautions are taken so that there is no variation of contact. If a sharp tap be