Page:Collected Physical Papers.djvu/329

Rh

The important conditions for securing quantitative results are (1) the maintenance of uniform intensity of stimulation in certain experiments and (2) the graduated increase of intensity in others.

Stimulus of torsional vibration.—I find that torsional vibration affords a 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, A rapid torsional to-and-fro vibration can 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. The plant chamber and the vibrational stimulator is shown in fig. 85.