tin-foil—separated by a sheet of non-conducting material, such as mica or paraffined paper. The capacity of the condenser is increased by enlarging the effective area of the plates. The diagram in fig. 9 illustrates this mode of excitation. By increasing the number of cells, the charging e.m.f. may be increased until a suitable value is obtained which is efficient for excitation. This will depend on the excitability of the plant-specimen. About 2 volts charging ·5 microfarad will in general be found sufficient. K is a special spring-key by which the condenser may be charged
Fig. 9.—Direct stimulation by condenser discharge; C, condenser, K, key. (a) intra-electrodal and (b) indirect extra-electrodal mode of stimulation.
or discharged. The plant to be excited is included in the electrical circuit. When K is pressed down, the condenser is charged, the instantaneous charging current passing in one direction. The upper arrow in the diagram shows the direction of this charging current. When the key is released, it springs back and discharges the condenser. The instantaneous discharge current now flows in a reverse direction (fig. 9).
The electrical connections with the plant are diagramatically shown, in this and other figures, by two lines. In practice the connections have to be made by means of thread moistened in dilute saline solution. In certain experiments it is necessary to avoid complications arising from electrolytic polarisation. In these it is advisable to
