INORGANIC RESPONSE--METHODS OF ENSURING CONSISTENT RESULTS
Preparation of wire--Effect of single stimulus.
I shall now proceed to describe in detail the response-curves obtained
with metals. The E.M. variations resulting from stimulus range, as has
been said, from ·4 volt to ·01 of that value, according to the metal
employed. And as these are molecular phenomena, the effect will also
depend on the molecular condition of the wire.
#Preparation of wire.#--In order to have our results thoroughly
consistent, it is necessary to bring the wire itself into a normal
condition for experiment. The very fact of mounting it in the cell
strains it, and the after-effect of this strain may cause irregularities
in the response.
For the purpose of bringing the wire to this normal state, one or all of
the following devices may be used with advantage. (1) The wires obtained
are usually wound on spools. It is, therefore, advisable to straighten
any given length, before mounting, by holding it stretched, and rubbing
it up and down with a piece of cloth. On washing with water, they are
now ready for mounting in the cell.
(2) The cell is usually filled with tap-water, and a period of rest
after making up, generally speaking, improves the sensitiveness. These
expedients are ordinarily sufficient, but it occasionally happens that
the wire has got into an abnormal condition.
[Illustration: FIG. 58.--EFFECT OF ANNEALING ON INCREASING THE RESPONSE
OF BOTH A AND B WIRES (TIN)
Stimuli (vibration of 160°) applied at intervals of one minute.]
In this case it will be found helpful (3) to have recourse to the
process of annealing. For if response be a molecular phenomenon, then
anything that increases molecular mobility will also increase its
intensity. Hence we may expect annealing to enhance responsiveness. This
inference will be seen verified in the record given in fig. 58. In the
case under consideration, the convenient method employed was by pouring
hot water into the cell, and allowing it to stand and cool slowly. The
first three pairs of responses were taken by stimulating A and B
alternately, on mounting in the cell, which was filled with water. Hot
water was then substituted, and the cell was allowed to cool down to
its original temperature. The six following pairs of responses were then
taken. That this beneficial effect of annealing was not due to any
accidental circumstance will be seen from the fact that _both_ wires
have their sensitiveness equally enhanced.
(4) In addition to this mode of annealing, both wires may be
short-circuited and vibrated for a time. Lastly (5) slight stretching
_in situ_ will also sometimes be found beneficial. For this purpose I
have a screw arrangement.
By one or all of these methods, with a little practice, it is always
possible to bring the wires to a normal condition. The responses
subsequently obtained become extraordinarily consistent. There is
therefore no reason why perfect results should not be arrived at.
[Illustration: FIG. 59.--UNIFORM RESPONSES IN TIN]
#Effect of single stimulus.#--The accompanying figure (fig. 59) gives a
series, each of which is the response curve for a single stimulus of
uniform intensity, the amplitude of vibration being kept constant. The
perfect regularity of responses will be noticed in this figure. The wire
after a long period of rest may be in an abnormal condition, but after a
short period of stimulation the responses become extremely regular, as
may be noticed in this figure. Tin is, usually speaking, almost
indefatigable, and I have often obtained several hundreds of successive
responses showing practically no fatigue. In the figure it will be
noticed that the rising portion of the curve is somewhat steep, and the
recovery convex to the abscissa, the fall being relatively rapid in its
first, and less rapid in its later, parts. As the electric variation is
the concomitant effect of molecular disturbance--a temporary upset of
the molecular equilibrium--on the cessation of the external stimulus,
the excitatory state, and its expression in electric variation,
disappear with the return of the molecules to their condition of
equilibrium. This process is seen clearly in the curve of recovery.
Different metals exhibit different periods of recovery, and this again
is modified by any influence which affects the molecular condition.
That the excitatory state persists for a time even on the cessation of
stimulus can be independently shown by keeping the galvanometer circuit
open during the application of stimulus, and completing it at various
short intervals after the cessation, when a persisting electrical
effect, diminishing rapidly with time, will be apparent. The rate of
recovery immediately on the cessation of stimulus is rather rapid, but
traces of strain persist for a short time.