Dixon and Poole — Photosynthesis and the Electronic Theory. 71 



by suddenly applying a known pressure of a few volts to the grid was found. 

 The grid was earthed, and the plate system isolated, and a reading of the 

 spot of light was taken. The grid was then raised to a known small pressure. 

 The induced charge caused a motion of the electrometer, which was again 

 read as soon as it had become steady. The grid was then earthed, thus 

 causing the spot of light to return nearly to its original position. The 

 difference between the second reading and the mean of the first and third 

 measures the rise in potential of the plate and quadrants for a known rise 

 of the grid. 



The entire system may be regarded as two condensers of capacities K] 

 and Kj in series. Ki is the capacity of the condenser formed by the grid and 

 the plate, and Kj that of all the rest of the system, including the lateral 

 capacity of the plate and wires, and the effective capacity of the electro- 

 meter quadrants. If v is the applied voltage, and E, the pressure indicated 

 by the electrometer, evidently (v-Ei) k, = EK2, since the charges on two 



condensers in series must be equal. Hence — -' = — . Since we can 



Ki E] 



calculate Ki approximately, we can find Ki + k,, which is the capacity of 

 the system when the grid is at a fixed potential, as in the actual test. As, 

 however, Ki can only be found rather roughly, this method is not accurate, 

 so the readings were repeated with an air-condenser of known capacity K3 

 connected in parallel with the electrometer, whose voltage was now found to 

 rise to E2. Hence 



Ki + K2 + K3 V Ki + K2 + K3 E,, E2K, 



= - ; so = — , or K, + K, . 



K, Ej Ki + Ko E2 E, - E2 



By this method the effective capacity of the system was found to be about 



167 cms. On this occasion the sensitivity of the electrometer was, for small 



motions, about 372 scale divisions per volt, so that one scale division 



167 

 corresponded to 77777; — rrwc,, or 1'50 x 10"^ E. s. u. of charge. Hence one 

 ^ 300 X 372 * 



scale division per minute represents a current of 2*5 x IQ-' e.s.u., or about 

 8 X 10"'^ ampere. This will not be very much affected by small variations in 

 the voltage of the electrometer needle, as the large effective capacity of the 

 electrometer is chiefly due to inductive effects caused by the motion of the 

 charged needle, and a small change in the charge on the latter will affect 

 the sensitivity (which increased with the potential of the needle) and the 

 capacity to about the same extent. 



In making a tes.t the plate system was isolated and the lamp turned on, 

 the beam of light being cut off by a cardboard shutter. As soon as the electro- 

 meter had become steady near its zero position, it was read and a stop-watch 



