ackground.

#Compensator.#--As the responses are on _variation_ of current of injury,
and as the current of injury may be strong, and throw the spot of light
beyond the recording surface, a potentiometer balancing arrangement may
be used (fig. 9), by which the P.D. due to injury is exactly
compensated; E.M. variations produced by stimulus are then taken in the
usual manner. This compensating arrangement is also helpful, as has been
said before, for calibrating the E.M. value of the deflection.

[Illustration: FIG. 9.--THE COMPENSATOR
A B is a stretched wire with added resistances R and R'. S is a storage
cell. When the key K is turned to the right one scale division =
.001 volt, when turned to the left one scale division = .01 volt. P
is the plant.]

#Means of graduating the intensity of stimulus.#--One of the necessities
in connection with quantitative measurements is to be certain that the
intensity of successive stimuli is (1) constant, or (2) capable of
gradual increase by known amounts. No two taps given by the hand can be
made exactly alike. I have therefore devised the two following methods
of stimulation, which have been found to act satisfactorily.

[Illustration: FIG. 10.--THE SPRING-TAPPER]

#The spring-tapper.#--This consists (fig. 10) of the spring proper (S),
the attached rod (R) carrying at its end the tapping-head (T). A
projecting rod--the lifter (L)--passes through S R. It is provided with
a screw-thread, by means of which its length, projecting downwards, is
regulated. This fact, as we shall see, is made to determine the height
of the stroke. (C) is a cogwheel. As one of the spokes of the cogwheel
is rotated past (L), the spring is lifted and released, and (T) delivers
a sharp tap. The height of the lift, and therefore the intensity of the
stroke, is measured by means of a graduated scale. We can increase the
intensity of the stroke through a wide range (1) by increasing the
projecting length of the lifter, and (2) by shortening the length of
spring by a sliding catch. We may give isolated single taps or superpose
a series in rapid succession according as the wheel is rotated slow or
fast. The only disadvantage of the tapping method of stimulation is that
in long-continued experiment the point struck is liable to be injured.
The vibrational mode of stimulation to be presently described labours
under no such disadvantage.

#The electric tapper.#--Instead of the simple mechanical tapper,