282 PROF. J. C. BOSE ON ELECTRIC RESPONSE IN 
is 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 stimu- 
lation, (1) by taps, (2) by vibration, both of which have been 
found to act satisfactorily. 
Spring-Tapper.—This consists of an arrangement by which a 
tapping-head in connection with a spring is lifted by means of 
the spokes of a cog-wheel, and then allowed to fall and strike 
the plant (fig. 5). The height of the lift, and therefore the 
intensity of the stroke, can be measured by means of a graduated 
scale. 
Fig. 5. 
Spring-Tapper. 
Vibrational Stimulus.—I find that torsional vibration affords 
another very effective method of stimulation. The plant-stalk 
may be fixed at one end, the other end being held in a tube 
provided with clamping jaws (figs. 6a and 20). A rapid 
torsional to-and-fro vibration may now be imparted to the 
stalk by means of the handle H. The amplitude of vibration, 
which determines the intensity of stimulus, can be accurately 
measured by means of a graduated circle. 
Intensity of Stimulus dependent on Amplitude of Vibration. 
I shall now describe an experiment which shows that torsional 
vibration is as effective as stimulation by taps or tbe blow 
delivered by a falling weight, and that the stimulating-intensity 
increases, length of stalk being constant, with the amplitude of 
vibration. I took a leaf-stalk of turnip and fixed it in the 
