ND B]

Usually I interpose an external resistance varying from one to five
megohms according to the sensitiveness of the wire. The resistance of
the electrolyte in the cell is thus relatively small, and the
galvanometer deflections are proportional to the E.M. variations. It is
always advisable to have a high external resistance, as by this means
one is not only able to keep the deflections within the scale, but one
is not troubled by slight accidental disturbances.

#Graduation of intensity of stimulus.#--If now a rapid torsional vibration
be given to A or B, an E.M. variation will be induced. If the amplitude
of vibration be kept constant, successive responses--in substances
which, like tin, show no fatigue--will be found to be absolutely
identical. But as 'the amplitude of vibration' is increased, response
will also become enhanced (see Chap. XV).

[Illustration: FIG. 57.--TOP VIEW OF THE VIBRATION CELL
The amplitude of vibration is determined by means of movable stops S S',
fixed to the edge of the graduated circle G. The index arm I plays
between the stops. (The second index arm, connected with B, and the
second circle are not shown.)]

Amplitude of vibration is measured by means of the graduated circle
(fig. 57). A projecting index, in connection with the vibration-head,
plays between fixed and sliding stops (S and S'), one at the zero point
of the scale, and the other movable. The amplitude of a given vibration
can thus be predetermined by the adjustment of the sliding stop. In this
way we can obtain either uniform or definitely graduated stimuli.

#Considerations showing that electric response is due to molecular
disturbance.#--The electromotive variation varies with the substance.
With superposition of stimuli, a relatively high value is obtained in
tin, amounting sometimes to nearly half a volt, whereas in silver the
electromotive variation is only about .01 of this value. The intensity
of the response, however, does not depend on the chemical activity of
the substance, for the electromotive variation in the relatively
chemically inactive tin is greater than that of zinc. Again, the sign of
response, positive or negative, is sometimes modified by the molecular
condition of the wire (see Chap. XII).

As regards the electrolyte, dilute NaCl solution, dilute solution of
bichromate of potash &c. are normal in their action, that is to say, the
electric response in such electrolytes is practically