METHODS OF STIMULATION 33 



(figs. 20, 21), showing the relative ineffectiveness of the 

 make-shock compared with the break-shock. 



Excitation by Tetanising Shocks 



It will be shown later that shocks individually ineffec- 

 tive become effective by repetition. It is thus possible to 

 excite a plant by subjecting it to a number of relatively 

 feeble make- and break-shocks. The advantage of this 

 mode of stimulation is that, owing to the low intensity of 

 these shocks, the liability of the tissue to injury is very 

 much reduced. Such alternating tetanising shocks can be 

 produced by means of an automatic spring-interrupter, 

 included in the primary circuits. This interrupter consists of 

 a steel spring carrying at its free end a soft-iron armature 

 which faces one pole of the electro-magnet of the primary. 

 An adjustable contact-rod touches the spring and com- 

 pletes the primary circuit. But the completion of the 

 circuit magnetises the electro-magnet, which, pulling the 

 armature, breaks the contact, thereby interrupting the 

 primary current. The electro-magnet is thus demagnetised, 

 the armature is released, and the spring returns suddenly, 

 re-establishing the circuit. By this automatic make-and- 

 break we obtain alternating induction-currents in the 

 secondary. 



When we wish to subject the experimental tissue to the 

 additive effects of these shocks, of a given short duration, 

 this may be accomplished by including in the primary 

 circuit a metronome, which in the course of a single beat 

 closes the circuit for an approximately definite duration. 

 If the duration of closure be, say, one-fifth of a second, and 

 if the frequency of the spring-interrupter be 50 times per 

 second, the number of alternating double-shocks given to 

 the tissue would be ten. 



A method of securing still greater accuracy in the 

 duration of the tetanising shock will be described in another 

 chapter, where also may be seen records obtained by that 



