48 COMPARATIVE ELECTRO-PHYSIOLOGY 



The next point is to secure an automatic arrangement 

 by which galvanometric connections can be made with the 

 experimental tissue, for a short period of time, say, 'Oi of a 

 second. In order to study the growing electro-motive changes, 

 these short-lived contacts are to be effected in successive 

 experiments at gradually increasing intervals of *oi, '02, -03 

 seconds, and so on after stimulation. It should be remem- 

 bered in connection with this subject that the reactions in 

 plant tissues are much more sluggish than those in the 

 animal. The time-intervals here provided for, therefore, are 

 even smaller than would have been strictly necessary,. 



The general plan of the apparatus for carrying out this 

 investigation is seen in fig. 37. The revolving rheotome-disc 

 carries two striking-bars, A and B, of which A is fixed, and 

 B capable of an increasing angular adjustment behind A. 

 The bar A, striking against the key K A , interrupts the electro- 

 magnetic circuit E, thus causing stimulation. All this time, 

 the galvanometer G is short-circuited by key K 2 , and it is 

 only when the striker B unshunts the galvanometer, by 

 striking against K 2 , that the responsive current can act on the 

 galvanometer. C is the compensating potentiometer, the 

 object of which will be described presently. The rheotome- 

 disc is rotated by means of a motor, provided with a perfect 

 governor, the period of a single rotation being adjusted to 

 one second. The circumference of the disc is 100 cm. One 

 centimetre of this circumference therefore represents an in- 

 terval of time of 'Oi second. The breadth of the striker B is 

 also i cm. and it will therefore pass over a given point in the 

 course of *oi second. These striking-rods attached to the 

 disc, impinge, as already said, against two electrical keys K,, 

 and K 2 which are adjusted along the same radius of the disc. 

 K! is a balanced key, one end of which carries a two-pronged 

 brass fork, both prongs of which are normally dipped in cups 

 of mercury, thus completing the particular electric circuit. 

 By the blow given by the striker A on a projecting rod 

 attached to Kj, this fork is tilted upwards, and the circuit is 

 broken. The striker B then impinges on the second key, K 2 . 



