(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 ineffective 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 completes 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
