116 Dr. J. Erskine-Murray. 



Darnell cell in the direction opposite to that given by the experi- 

 mental plates. The temporary connection is now broken and the 

 plates are separated. In doing so the capacity of the condenser is 

 reduced, hence if there be any electric charge on them it will be 

 indicated by a further deflection of the electrometer. This operation 

 is repeated with different values of the counterpotential until separa- 

 tion of the plates produces no change in the deflection of the 

 electrometer. Since it has annulled the charge, the counter- 

 potential must now be equal and opposite to the natural potential- 

 difference. 



5. During most of the experiments a piece of apparatus made 

 by Lord Kelvin in or about 1861 was used. In it the plates were 

 surrounded by a cylindrical zinc case. This was made in two parts 

 for convenience in manipulation, the lower fixed to a heavy cast-iron 

 base plate, while the upper stood upon small brackets attached to 

 the top edge of the lower part, its position being regulated by a 

 hole-slot-and-plane arrangement. Openings in the sides of the 

 upper part facilitated the adjustment of the experimental plates, but 

 these were closed by a sliding cover during a measurement of 

 potential. 



The lower plate was supported by an insulating glass stem which 

 was kept dry by means of pumice and sulphuric acid. On the top of 

 this stem was a brass cap screwed to fit the sockets which were 

 soldered to the backs of the plates, and from the brass cap a stiff 

 wire passed out through a hole in the case and formed the connection 

 between fche insulated plate and the electrometer. 



A piece of platinum foil was soldered round a part of this wire to 

 give a clean contact for the temporary connection between the plates 

 through the potential-divider, and the wire from the divider had 

 likewise a platinum end piece. 



The upper plate hung, by a ring at the centre of its back, on a 

 hook at the lower end of a vertical metal rod, which could be drawn 

 up so as to increase the distance between the plates. A small disc 

 fixed on the rod at right angles to it just above the hook had three 

 screws passed through it and pressed against the plate, holding it 

 firmly in the hook ; by this means the upper plate can be set parallel 

 to the lower one in a very short time. This apparatus, which is the 

 same as that used by Lord Kelvin in his experiments many years 

 ago, was found to be very convenient, the arrangements enabling us 

 to take out and replace a plate very rapidly, a matter of great 

 importance when observing temporary variations, while at the same 

 time both plates were firmly attached to their supports when in 

 position. 



6. The divider, whose total resistance was about 2400 ohms, 

 was made to divide the potential-difference between its outer 



