30 Mr. T. Andrews. Electromotive Force from [May 8, 



Method of Experimentation. 



The experiments were conducted as follows, in precisely the same 

 manner in each case for comparison. 



For the purposes of this research the cells were constructed so that 

 the diffusion effects, electromotive force, <fcc., observed should 

 approximate to those obtaining during a tidal period of six hours. 



A strong wooden box was divided into two equal compartments, A 

 and B, the partition containing at the 'lower end a porous diaphragm 

 of chamois leather to allow of a suitable diffusion between the 

 solutions. 



Two bars from the same piece of metal (of precisely the same 

 composition), polished bright, and exactly 2f- inches diameter, were 

 placed in the cells exactly at equal distances apart in each case (1W 

 inch), one bar in partition A, and the other bar in partition B, the 

 bars being attached to the galvanometer No. 1. The partitions A 

 and B were then simultaneously carefully filled up to a depth of 12 

 inches, the one with sea water, the other with distilled water, and 

 careful telescopic readings taken of the time changes in the deflections 

 of the galvanometer No. 1, regularly during the tidal period of six 

 hours. 



The difference in level between the solutions in the cells, caused by 

 the greater specific gravity of the sea water, assisted diffusion, thus 

 approximating to the current pressures in rivers exerted by the tidal 

 flow. 



To render the application of these experiments as practical as 

 possible, the observations of the electromotive force and time changes 

 of resistance during diffusion, were taken at regular intervals of two 

 and a half minutes (the results being summarised in Table B), over 

 tidal periods of six hours, so as to afford an approximation to the 

 effects produced by alternating diffusion of salt and fresh water 

 during the tidal changes of a tidal river. 



The galvanometer No. 1 with its accessories, resistance coils, Ac., 

 employed in these experiments was a delicate astatic one (by Messrs. 

 Elliott Bros.), suspended needles, large mirrored dial, and it was also 

 arranged to work as a mirror galvanometer (R. of galvanometer 521 

 ohms at 20 C.). 



It was constructed as to resistance, &c., specially to suit the 

 purpose of this research. 



The galvanometer was carefully calibrated throughout, on the spot, 

 at the commencement of the research with a Daniell's cell in circuit, 

 and the constancy of the instrument afterwards frequently verified. 



Another astatic galvanometer, No. 2 (suspended needles), of lower 

 resistance was used for taking the time changes in the resistance of 

 the cells, by the first fling method as described further on. 



