re A B, the upper part of the
circuit being directly connected by the electrolyte. Vibration of A will
now give rise to a current of response which flows in the metallic part
of the circuit with the interposed galvanometer from B to A. We see that
though the direction of the current in this is the same as in the last
case, yet the galvanometer deflection is now reversed, for the evident
reason that we have it interposed in the metallic and not in the
electrolytic part of the circuit.

The next arrangement (_d_) consists simply of the preceding placed
upside down. Here A and B are held parallel to each other in an
electrolytic bath (water). Mechanical vibration may now be applied to A
without affecting B, and _vice versa_.

The actual apparatus, of which this is a diagrammatic representation, is
seen in (_e_).

Two pieces, from the same specimen of wire, are clamped separately at
their lower ends by means of ebonite screws, in an L-shaped piece of
ebonite. The wires are fixed at their upper ends to two
electrodes--leading to the galvanometer--and kept moderately and
uniformly stretched by spiral springs. The handle, by which a torsional
vibration is imparted to the wire, may be slipped over either electrode.
The amplitude of vibration is measured by means of a graduated circle.

It will be seen from these arrangements:

(1) That the cell depicted in (_e_) is essentially the same as that in
(_a_).

(2) That the wires in the cell being immersed to a definite depth in the
electrolyte there is always a perfect and invariable contact between the
wire and the electrolyte. The difficulty as regards variation of contact
is thus eliminated.

(3) That as the wires A and B are clamped separately below, we may
impart a sudden molecular disturbance to either A or B by giving a quick
to-and-fro (torsional) vibration round the vertical wire, as axis, by
means of the handle. As the wire A is separate from B, disturbance of
one will not affect the other. Vibration of A produces a current in one
direction, vibration of B in the opposite direction. Thus we have means
of verifying every experiment by obtaining corroborative and reversed
effects. When the two wires have been brought to exactly the same
molecular condition by the processes of annealing or stretching, the
effects obtained on subjecting A or B to any given stimulus are always
equal (fig. 56).

[Illustration: FIG. 56.--EQUAL AND OPPOSITE RESPONSES EXHIBITED BY A
A