544 COMPARATIVE ELECTRO-PHYSIOLOGY 



was now made the fourth arm of a Wheatstone's bridge 

 (fig. 327), by which electrical resistance is usually determined. 

 The plant specimens employed generally possessed high 

 resistance, of the order of several hundreds of thousands of 

 ohms. In the Wheatstone's arrangement employed by me, 

 p and Q represented the ratio-arms ; R a standard megohm, 



FIG. 327. Diagrammatic Representation of Experimental Arrange- 

 ment for Recording Response by Resistivity Variation 



p Q, ratio arms of Wheatstone's bridge ; R, standard i or -5 megohm ; 

 s, specimen. 



or half-megohm ; and s the specimen whose variations of 

 resistance were to be determined. It is now evident that 



when the bridge is balanced, S = - R. 



The ratio-arms, P and Q, consist of resistance-boxes, 

 which allowed a variation of from I to 10,000 ohms. In 

 order to obtain balance, of course, the ratio of the two had to 

 be suitably adjusted. A highly sensitive galvanometer was 

 used, and the electromotive force employed to obtain balance 

 was only '05 volt. This low E.M.F. was obtained by the 

 use of a suitable potentiometer slide. It will be seen that, 

 owing to the very low E.M.F. and the high resistance in 

 the circuit, the current flowing through the specimen was 

 rendered extremely feeble. This was done in order to avoid 



