50). Any other metal could be used instead of tin.

#Experiment on tin, block method.#--Let us then take a piece of tin
wire[15] from which all strains have been previously removed by
annealing, and hold it clamped in the middle at C. If the strains have
been successfully removed A and B will be found iso-electric, and no
current will pass through the galvanometer. If A and B are not exactly
similar, there will be a slight current. But this will not materially
affect the results to be described presently, the slight existing
current merely adding itself algebraically to the current of response.

If we now stimulate the end A by taps, or better still by torsional
vibration, a transitory 'current of action' will be found to flow in the
wire from B to A, from the unstimulated to the stimulated, and in the
galvanometer from the stimulated to the unstimulated. Stimulation of B
will give rise to a current in an opposite direction.

[Illustration: FIG. 50.--ELECTRIC RESPONSE IN METALS
(_a_) Method of block; (_b_) Equal and opposite responses when the ends
A and B are stimulated; the dotted portions of the curves show
recovery; (_c_) Balancing effect when both the ends are stimulated
simultaneously.]

#Experiment to exhibit the balancing effect.#--If the wire has been
carefully annealed, the molecular condition of its different portions is
found to be approximately the same. If such a wire be held at the
'balancing point' (which is at or near the middle) by the clamp, and a
quick vibration, say, of 90 deg. be given to A, an upward deflection will be
produced; if a vibration of 90 deg. be given to B, there will be an equal
downward deflection. If now both the ends A and B are vibrated
simultaneously, the responsive E.M. variation at the two ends will
continuously balance each other and the galvanometer spot will remain
quiescent (fig. 30, A, B, R). This balance will be still maintained when
the block is removed and the wire is vibrated as a whole. It is to be
remembered that with the length of wire constant, the intensity of
stimulus increases with the amplitude of vibration. Again, keeping the
amplitude constant, the intensity of stimulus is increased by shortening
the wire. Hence it will be seen that if the clamp be shifted from the
balancing point towards A, simultaneous vibration of A and B through 90 deg.
will now give a resultant upward deflection, showing that the A response
is now relatively stronger. Thu