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The series in fig. 75 explains the transformation from the "straight wire" to "cell" form. The wires A and B, cut from the same piece, are clamped separately below; vibration of A (the amplitude of which is measured by a graduated circle) gives rise to a responsive current in one direction; vibration of B produces a current in the opposite direction. Every experiment can thus be verified by corroborative and reversal effects. The intensity of electromotive response varies with the substance, and is sometimes considerable, for example, with "tin," a single vibration may give rise to as high a value as 0.4 volt. The intensity of response does not depend on the chemical activity of the substance, for the electromotive variation in the relatively inactive tin is greater than that in zinc. Again, the sign of response, positive or negative, is sometimes modified by the molecular condition of the wire (see below). In the cell form of apparatus the wires are immersed to a definite depth in the electrolyte; there is thus a perfect and invariable contact between the wire and the electrolyte. The wire is clamped below, and torsional vibration gives rise to a strong electrical response. If the wire be carefully unclamped, vibration is found to cause no electrical response. As all the rest of the circuit is kept absolutely the same in the two different sets of experiments, the results offer conclusive proof that the responsive electromotive variation is solely dire to the mechanical stimulation of the acted wire. The excitatory effect due to stimulation persists for a time. This is demonstrated by keeping the galvanometer circuit open during the application of stimulus, and completing it at various short intervals