#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, an electromagnetic tapper may be used. [Illustration: FIG. 11.--THE TORSIONAL VIBRATOR Plant P is securely held by a vice V. The two ends are clamped by holders C C'. By means of handles H H', torsional vibration may be imparted to either the end A or end B of the plant. The end view (_b_) shows how the amplitude of vibration is predetermined by means of movable stops S S'.]