Page:Researches on Irritability of Plants.djvu/89

66 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

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).

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