452 
PROFESSOR W. WATSON ON A DETERMINATION OF THE VALUE OF 
advantage in this respect. The advantage of the porous pot would j^robably be much 
greater in those cases where a larger weight of silver per cub. centim. of the solution was 
deposited during the electrolysis. The porous pot voltameter has one disadvantage 
compared with the Rayleigh pattern in that its resistance is considerable, and that 
this resistance, unless suitable precautions are taken, will vary very considerably. 
Of course this change of resistance will not be of any consequence, when we simply 
wish to compare the weights of silver dej)osited in two voltameters placed in series. 
More frequently, however, the problem is to use the voltameter to measure a current, 
so that changes in the resistance of the circuit, accompanied as they must be by 
changes in current, are objectionable. If the porous pot used in the voltameter is dry 
when inserted in the voltameter or has been soaking in water, then for the first hour 
the resistance of the voltameter will decrease very considerably and irregularly. 
Thus, it was found that the increase in the resistance which had to be made to keep 
the current in the circuit constant, even when the total resistance of the circuit 
amounted to over 120 ohms, was so great as to make it almost impossible to maintain 
a proper balance in the potential circuit. Further it was impossible to adjust a 
balancing’ resistance, to be switched out of the circuit when the voltameter was 
switched in, so that no harmful change ot current took place at the start of a 
deposition. 
The above objections were to a great extent got over by keeping the porous pots, 
when not in use, in a jar of the same solution as that used in the voltameters. Thus 
the pores of the porous jjot were filled with the solution even at the commencement 
of a deposition. The balancing resistance w’as generally so well adjusted that on 
switching in the voltameters the current did not differ from its proper value by more 
than 1 part in 10,000, and even this small want of balance could be set right by 
altering the carbon resistances in a few seconds. The current was kejDt constant 
throughout the deposition by means of these same carbon resistances, which as long 
as the plates are not too slack, are to all intents and purposes perfect in the way 
they behave. 
In the following table are given the w^eights of silver obtained in the various 
experiments, some of which were made before and some after the magnetic observa¬ 
tions. In order to be able to compare the different measurements the E.M.F. of the 
cadmium cell reduced to 20° as calculated from the weight of silver deposited, 
assuming the electro-chemical equivalent of silver as 0'001118, is given in each case. 
