EXPERIMENTAL ARRANGEMENT 317 



— 90°, the side B being now the upper ; the induced 

 galvanometric negativity of that side gave down-response, 

 which disappeared on return to the zero position (fig. 192). 



Experimental Arrangement 



I give a more detailed account of the experimental 

 method. The sensitiveness of the galvanometer for the 

 record should be such that a current of io~'° amp. gives a 

 deflection of i mm. at a distance of a metre. 



Non-polarisahle electrodes. — -The electric connections 

 with the plant are usually made by means of non-polarisable 

 electrodes (amalgamated zinc rod in zinc-sulphate solution 

 and kaohn paste with normal saline). I at first used this 

 method and obtained all the results which will be presently 

 described. But the employment of the usual non-polarisable 

 electrodes w^ith liquid electrolyte is, for the present purpose, 

 extremely inconvenient in practice ; for the plant-holder 

 with the electrodes has to be rotated from vertical to hori- 

 zontal through 90°. The reliability of the non-polarisable 

 electrode, moreover, is not above criticism. The zinc- 

 sulphate solution percolates through the kaolin paste and 

 ultimately comes in contact with the plant, seriously affect- 

 ing its excitability. The term ' non-polarisable electrode ' 

 is in reality a misnomer ; for the action-current (whose 

 polarising effect is to be guarded against) is excessively 

 feeble, being of the order of a milhonth of an ampere or 

 even less ; the counter-polarisation induced by such a 

 feeble current is practically negligible. 



The idea that non-polarisable electrodes are meant to 

 get rid of polarisation is thus not justified by the facts of 

 the case. The real reason for its use is very different. 

 The electric connection with the plant has to be made 

 ultimately by means of two metal contacts. If two pieces 

 of metal even from the same sheet be taken and put in con- 

 nection with the plant, a voltaic couple is produced owing 

 to slight physical differences between the two electrodes. 



