Page:Researches on Irritability of Plants.djvu/89

This page has been proofread, but needs to be validated.
66
RESEARCHES ON IRRITABILITY OF PLANTS

according to the season and the condition of the plant. In practice, therefore, we require arrangements by which successive stimulations can be automatically effected at these intervals. As we require a slowly moving plate for the purpose of these records, the plate-carrier is let down by a thread which is wound round a wheel attached to the minute-hand axis of the driving-clock. To the same axis is also screwed one or other of the three separate discs, bearing equidistant projecting rods, either 3, 4, or 6 in


Fig. 30.—Periodic Starter and Automatic Exciter.

number. During one complete revolution, which takes an hour, these rods will press and release a spring at intervals of 20, 15, or 10 minutes, as the case may be (fig. 30).


The Automatic Exciter


If the primary circuit of the induction coil provided with a spring-interrupter be closed for a definite period of time, say ·1 second, then the number of interruptions, with consequent induction-shocks, will also be definite. What is wanted is some contrivance for release, through which the Periodic Starter can close the main circuit for a definite length of time, say ·1 second. It might at first sight appear that this could be secured by an electrical contact made by the revolving radial-rods already referred to, but the