ve of molecular disturbance will reach A and
B. The mechanical and the attendant electrical disturbance will at these
points reach a maximum and then gradually subside. The resultant effect
in the galvanometer will be due to E_A-E_B when E_A and E_B are
the electrical variations produced at A and B by the stimulus. The
electric changes at A and B will continuously balance each other, and
the resultant effect on the galvanometer will be zero: (_a_) if the
exciting disturbance reaches A and B at the same time and with the same
intensity; (_b_) if the molecular condition is similar at the two
points; and (_c_) if the rate of rise and subsidence of excitation is
the same at the two points. In order that a resultant effect may be
exhibited in the galvanometer, matters have to be so arranged that the
disturbance may reach one point, say A, and not B, and _vice versa_.
This was accomplished by means of a clamp, in 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 exalted or depressed by physical or chemical means. We shall in
Chap. XVI study in detail the effect of chemical reagents in producing
the enhancement or depression of excitability. There are thus two other
means of obtaining a resultant effect--(2) by the method of relative
depression, (3) by the method of relative exaltation.

#Electric response by method of depression.#--We may thus by reducing or
abolishing the excitability of one end by means of suitable chemical
reagents (so-called method of injury) obtain response in metals without
a block. The entire length of the wire may then be stimulated and a
resultant response will be produced, owing to the difference between the
excitability of the two ends. A piece of tin wire is taken, and one
normal contact is made at A (strip of cloth moistened with water, or
very dilute salt solution). The excitability of B is depressed by a few
drops of strong potash or oxalic acid. By the application of the latter
there will be a small P.D. between A and B; this will simply produce a
displacement of zero. By means of a potentiometer the galvanometer spot
may be brought back to the original position. The shifting of the zero
will not affect the general result. The effect of mechanical stimulus is
to produce a transient electro-motive response, which will be superposed
algebraically on the existing P.D. The deflect