96 RESPONSE IN THE LIVING AND NON-LIVING 
processes of annealing or stretching, the effects obtained 
on subjecting A or B to any given stimulus are always 
equal (fig. 56). 
Usually I interpose an external resistance varying 
from one to five megohms according to the sensitive- 
ness 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 imduced. 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 vibra- 
Fic. 57.—Tor View or rae Visna- tion’ 1s increased, response 
poet will also become enhanced 
The amplitude of vibration is deter- 
mined by means of movable stops fee 
8S’, fixed to the edge of the gra- (see Chap. AV): 
duated circle G. The index arm : | =o 
I plays between the stops. (The Amplitude of vibration 
second index arm, connected with 
B, and the second circle are not 
shown.) 
is measured by means of the 
eraduated circle (fig. 57). A 
projecting index, in connection with the vibration-head, 
plays between fixed and sliding stops (S and $’), one 
at the zero point of the seale, and the other movable. 
