42 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
gradually brought nearer to the primary, it is observed that no effect whatever follows 
the passage of the induction current until the secondary coil is approximated to a 
relatively short distance, usually about 10 centims. from the primary. (2) That the 
excitatory effect produced at this distance is unmodified by further approximation. 
(3) That the distance of the secondary coil at which the first response takes place, 
when in a succession of excitations it is gradually approximated, is much greater 
when the induction current is directed from the upper to the under surface than in 
the contrary case; in other words, that the leaf responds to a weaker current when 
it is directed downwards than when it is directed upwards; and, finally (4) That if 
one current of an intensity a little less than sufficient to evoke a response be followed 
at a short interval of time by a second current of similar intensity, the latter may 
be effectual. This last fact will be reverted to further on, under the head of 
“ Summation.” 
Excitatory influence of the voltaic current. 
For the purpose of studying the influence of a weak voltaic current, a rheochord of 
platinum wire was used, of which the length was 14 feet and the resistance 20 ohms; 
this was interpolated in the circuit of a Daniell cell. Currents could be derived 
from it and conducted through the leaf by connecting one electrode with the end of 
the rheochord, the other with any point of its length by a slider. In this case the 
derived current might, in consequence of the relatively high resistance of the plant, be 
regarded as proportional to the length of wire between the end block and the slider. 
By comparing this rheochord with the compensator it was found that each foot of wire 
corresponded to a tension difference of about 0'06 D. and hence the whole wire to about 
0’84 D. With this contrivance the following observations were made. In the first 
series the leaf was led off to the galvanometer as in the fundamental experiment, but 
a switch (a Poht/s Wippe, without cross wires) was introduced into the circuit, of such 
construction that for one or more seconds a derived current from the rheochord could 
be sent through the electrodes, the galvanometer circuit being for the same period 
broken. As care was taken to compensate immediately before the switch was thrown 
over, this produced no movement of the needle, but served to show in what way 
the passage of the current modified the electromotive properties of the leaf. The 
effects observed were as follows:—(1) When a very weak current was used (l foot of 
wire, corresponding to a difference of potential of 0*06 D.) a slight effect, resembling 
polarization, was observed— i.e., a transitory deflection indicating the existence in the 
leaf of a current opposed to the current which had been led through it, its direction 
being upwards when the anode was on the upper surface, and vice verscl It was 
ascertained that this was not due to polarization of the surfaces of contact of zinc 
with zinc sulphate solution, for when the experiment was repeated with the clay 
points in contact with each other, i.e., without the interposition of the leaf, no 
