450 PROFESSOR W. WATSON ON A DETERMINATION OF THE VALUE OF 
the kathode, Richards, Collins, and Heimrod have devised a new form of volta¬ 
meter in which the anode is surrounded by an earthenware j)orous pot, the level 
of the solution being kept lower inside the pot than outside. They find that this 
porous pot form of voltameter consistently gives values for the electro-chemical 
equivalent, which are 0‘082 per cent, lower than the values given by the ordinary 
Rayleigh form of voltameter. 
As this new form of voltameter apjieared as if it might offer some advantages 
over the Rayleigh pattern, two voltameters were always placed in the cmcuit when 
making the measurement, one being a porous pot voltameter, and the other a 
Rayleigh voltameter. 
The porous pot voltameter was a close copy of that originally used by the inventors, 
and is shown in fig. 6. The platinum basin A rested 
in a circular hole cut in a piece of sheet copper attached 
to an ebonite base. The silver anode B was held in 
a spring clip attached to an arm carried by the metal 
upright C. A second arm D, carried a glass ring from 
which the porous jDot was suspended by platinum 
wires. When in use, the platinum basin contained 
about 75 cub. centims. of solution, and afforded a 
kathode surface of about 80 sq. centims. The anode 
was a rod of pure silver 1 centim. in diameter, and 
the anode surface was about 10 sq. centims. The 
silver from which the anodes were made in both forms 
of voltameter, was obtained from the Royal Mint 
through the kindness of Sir William Roberts-Austen, 
and Dr. T. K. Rose. 
The Rayleigh voltameter consisted of a platinum bowl 7'5 centims. in diameter, 
containing about 55 cub. centims. of solution, the kathode surface being about 
80 sq. centims. The anode consisted of a small silver plate with upturned corners, 
the anode surface being about 10 sq. centims. The anode was wrapped in a small 
Swedish filter paper. 
A solution containing 20 grammes of crystallized silver nitrate to 80 grammes of 
water was used. The solution was always tested, and found to be neutral to test 
paper. The same solution was never used in the voltameter twice, so that even at 
the end of a deposition the amount of silver deposited from the solution in the 
porous pot voltameter was only 0‘02 gramme per cub. centim., and in the Rayleigh 
voltameter, 0‘03 gramme per cub. centim. 
The weight of silver dejaosited was obtained by means of a Bunge balance fitted 
with a microscope to read the movements of the pointer. The sensitiveness of this 
balance was such, that when loaded with one of the platinum basins, a tenth of a 
milligramme produced a deflection of three scale divisions. The weights employed 
Fig. 6. 
