492 



Mr. W. Craig Henderson on 



longer exist, and a positive deflexion on the electrometer 

 scale will be obtained. 



Consider what would be the effect of the atmosphere in 

 the box becoming saturated with water-vapour. In such a 

 state evaporation still continues, only there is as much con-> 

 densation as evaporation. Thus if the evaporating particles 

 carry a charge they should be driven by the electric field 

 over to the plate C and there discharged, while the particles 

 condensing on the water-surface do not bring a charge with 

 them. But probably around these charged particles a rapid 

 condensation would take place, and some of them would be 

 returned to the water- vessel carrying their charge with them. 

 In order, therefore, to obtain the maximum effect the holes 

 in the side and lid of the box were large enough to prevent 

 saturation of the atmosphere within. 



To eliminate external effects experiments were made with 

 the vessel V empty as well as with V full of water. The 

 following are the results of four experiments : — 





Potential of C. 



Deflexion from zero of 

 Electrometer. 





+ 80 volts 

 + 80 „ 

 + 143 ,. 

 + 143 „ 



in 57 minutes 

 - 1 ;, 55 „ 

 + 10 „ 60 „ 

 + 9 „ 60 „ 



II. „ „ „ 



III. (a) Without water in V ... 

 (b) With „ „ ... 



In III. (a) the potential of the cells fell through 2 volts in 

 the hour during which readings were taken; while in III. (b) 

 it fell through 3 volts. It remained, therefore, to find the 

 effect on the deflexion of this fall of potential. To determine 

 this V was put to earth by joining the pairs of quadrants, C 

 was then raised to a known potential, and V then insulated. 

 The wire from C to the cells was then disconnected from the 

 cells and joined to the case A, and the deflexion on the elec- 

 trometer noted. The following readings were taken: — 



With +4 volts on C, when C was put to earth, deflexion = — 4 scale-divisions. 

 „ +20 „ „ „ „ ., „ „ =-20 



„ +40 „ „ ., „ „ „ „ =-40 



The inductive effect, therefore, on V of a fall of potential of 

 C is to give a negative deflexion on the electrometer scale 

 of 1 scale-division per volt fall of potential. To the deflexion 

 in experiment III. {a) we must therefore add 2, and to that 

 in III. (b) we must add 3. We thus get a deflexion of 12v 

 in each case. 



