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 •! second, then the number of interruptions, with con- 

 sequent 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 -i 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 



