208 RESEARCHES ON IRRITABILITY OF PLANTS 



by e, which is therefore the anode. It leaves by e', which 

 is therefore kathode. The advantage of the slide is that 

 the e.m.f. applied may be gradually increased from zero to 

 maximum, or decreased from maximum to zero. In certain 

 experiments, where we wish to apply a given e.m.f. without 

 causing excitation, this is essential. For we have seen 

 that a sufficiently graduated increase or decrease of current 

 causes no excitation. If excitation be desired, however, 

 it may be induced by a sudden variation of current brought 

 about by suddenly moving the slide either backwards or 



Fig. hi. — The Potential Slide. 



forwards, or by suddenly completing or interrupting the 

 main circuit (fig. III). 



It is important also to know the e.m.f. that has been 

 applied and the intensity of the current flowing through 

 the plant in a given experiment. For the former, a scale 

 fixed on the apparatus may be previously calibrated so as to 

 give the values of the e.m.f. at the different positions of the 

 sliding-contacts, it being understood that the same number 

 of storage-cells are always applied at the terminals. Or we 

 may place a voltmeter to measure the applied e.m.f. between 

 the two sliding-contacts. 



For the measurement of the currents that flow through 

 the plant, a micro-amperemeter is used. Each division 



