THE LEAF CONSIDERED AS AN ELECTRIC ORGAN 251 



following each other in rapid succession. For the resultant 

 negative polarisation will in practice be neutralised, if the 

 primary polarising currents are similar, equal, and opposite. 

 The stimulus applied on the two surfaces, moreover, will be 

 equal, if the two rapidly succeeding and opposite-directioned 

 shocks be so symmetrical as to be interchangeable. Which- 

 ever may be the factor of excitation will then act equally on 

 both surfaces. The response, therefore, will now be determined 

 solely by the natural difference of excitability as between the 

 two surfaces. 



It has been said that in order to accomplish these experi- 

 mental conditions, the two opposite shocks should be equal in 

 intensity and in point of time-relations. An ordinary make- 

 and-break RuhmkorrFs shock does not fulfil this condition, since 

 the break-shock is there the quicker and more intense of the 

 two. Moreover, owing to the varying residual magnetisation 

 in the iron core, successive shocks may not be equal. These 

 defects are overcome by sending round the primary, with 

 a constant rapidity, two equal and opposite currents in 

 alternation. During one semi-cycle, then, the primary current 

 varies from + C to C, and during the next from C to 

 -1- C, and since these two changes are effected with the same 

 rapidity, the induced currents are symmetrical, equal, and 

 opposite. 



Such reversal of current is accomplished by means of a 

 rotating reversing-key. The key R is wound up against the 

 tension of a spring S, being maintained in this set position 

 by the electro-magnet E, acting on the armature. When the 

 current in the electro-magnet is broken, the alternating 

 double shock from the induction coil I is passed through the 

 experimental leaf L, by means of non-polarisable electrodes 

 N, N 2 . In the case just described the sequence of the 

 current through the primary coil was, say, right-left-right. 

 In the next experiment, by means of the Pohl's commutator, 

 K 4 , this sequence may be made left-right-left (fig. 159). 



Employing this method, I have carried out rheotomic 

 observations for determining the time-interval after the shock 



