408 Dr. J. Hopkinson and Messrs. E. Wilson and F. Lydall. 



half period increased, the potential difference would also increase, 

 until it attained the value given with a continuous current, and that 

 when this value was attained, the curve of potential and time would 

 exhibit a flat top for all higher numbers of coulombs passed. We 

 thought it possible that from the number of coulombs per unit of 

 section required to bring the potential difference to its full value, we 

 could obtain an idea of how thick a coating of the ions sufficed to 

 secure that the surface of the plate had the chemical quality of the 

 ion and not of the substance of the plate. 



Platinum Plates. 



Part I. In the first instance, two cells having platinum plates for elec- 

 trodes were used. We are indebted to Messrs. Johnson and Matthey 

 for the loan of these plates. They have each an area of 150 sq. cm. 

 exposed to one another within the electrolyte, and are placed in a 

 porcelain vessel J in. apart. Pieces of varnished wood were 

 placed at the back of each plate so as to prevent conduction between 

 the outside surfaces through the fluid. The solution used was of 

 water 100 parts by volume, and H 2 S0 4 5 parts. Fig. 1 gives a dia- 

 gram of connections, in which A, B are the terminals of a Siemens 



FIG. 1. 



W12 alternator, 0, C are the cells above described, in series with 

 which is placed a non-inductive resistance, DE. By means of a two- 

 way switch, F, one of Lord Kelvin's quadrant electrometers, Q, could 

 be placed across the cells C, C or the non-inductive resistance DE 

 through a revolving contact-maker,* K, fixed to the shaft of the alter- 

 nator. A condenser of about 1 m.f . capacity was placed across the 

 terminals of the electrometer. 



* For description of contact-maker see * Eoy. Soc. Proc.,' vol. 53, p. 357. 



