412 Mr. A. P. Laurie on Measurements of the 



Although it is well known that blowing air through the 

 liquid enormously increases the current from bichromate cells 

 (probably because it thoroughly mixes the liquid), I do not 

 know of any measurements having been made with cells with 

 moving plates ; and even if such measurements have been 

 made, additional results are nevertheless of interest. 



The arrangement I adopted was as follows : — A shallow 

 glass jar was lined inside with platinum foil to which pla- 

 tinum wires had been welded. This platinum-foil formed the 

 one electrode of the cell, and amounted to 90 square centi- 

 metres in surface. This jar was placed under a horizontal 

 wooden pulley with a deep groove cut in its upper surface. 

 A binding-screw was screwed into the pulley from below until 

 the end of the screw emerged in the bottom of the groove 

 above mentioned. To this binding-screw a little cadmium 

 plate was attached, having 10'6 square centim. of its surface 

 immersed in the liquid. (This is counting the surface of 

 both sides of the plate.) 



The groove in the pulley was half-filled with mercury, and 

 an amalgamated flattened copper wire, fixed to the framework 

 of the apparatus, dipped into the mercury in the groove. 

 In this way metallic connection was made with the cadmium 

 plate. The cadmium plate was driven through the liquid by 

 an old clock, altered for the purpose, at the rate of about two 

 or three revolutions a second*. 



In order to check a little the moving of the liquid, little 

 glass pillars were cemented at short intervals to the surface 

 of the platinum plate. The cell held about 100 cubic centim. 

 of liquid. In order to measrre the current from this cell it 

 was connected to the Helmholtz galvanometer in Prof. Taif s 

 laboratory. 



Taking into consideration the amount of accuracy with 

 which the constant of this instrument has been determined, 

 and the amount of accuracy possible in the readings of the 

 galvanometer-needle, I believe the following measurements of 

 E.M.F. may be taken as having a possible maximum error of 

 1 per cent. 



The method of experimenting was as follows : — 

 The cadmium plate was weighed, both before and after the 

 experiments, the loss of weight amounting to *8275 grm., and 

 the solution analyzed. Before weighing after the experi- 

 ments, the plate was washed in water, then in alcohol, and 



* This apparatus was a copy of a similar arrangement used by Dr. Gib- 

 son, of Edinburgh University, in some experiments on Electrolysis. 



