POLAR EFFECTS OF MODERATE CURRENTS 217 



of ebonite, slides up and down on a vertical stand and can 

 be fixed at any height by a screw. The piece of ebonite 

 carries two small plates of brass, each supporting a bored 

 cork through which slides a straight -form non-polarisable 

 electrode. The two pieces of brass can also be adjusted 

 laterally, so as to make the electrical connections with the 

 two sub-petioles at varying distances apart. The main 

 petiole is also supported mechanically and thereby kept 



Fig. 113. — ^The electrode-holder. 



from falling, by a strut attachment underneath, as shown 

 in the diagram, which represents two out of four sub-petioles 



(fig. 113). 



Having thus supported the leaf, the next point is to 

 make effective electrical contacts, with middle points on 

 the two neighbouring sub-petioles. In doing this, consider- 

 able difficulty is encountered by reason of the waxy coating 

 on the surface of midrib and leaflets. On this account there 

 is apt to be no proper electrical contact between the moist 

 thread of the electrode and the specimen. This obstacle may 

 be overcome, however, by directing a jet of very dilute solution 

 of ether or alcohol — either of which will dissolve wax — 



