1384.] Difference of Potential in Tidal Streams. 31 



The results of the observations giving the E.M.F., &c., during- 

 diffusion are recorded in the summary of results on Table B, the 

 E.M.F. being calculated from the ascertained resistances in circuit, in 

 conjunction with the known calibration of the galvanometer No. l r 

 which was used for this part of the research. 



For ascertaining the comparative behaviour of the various metals 

 employed, the author has given the average, together with the highest 

 E.M.F. noted during the observations in each case. 



Determination of Resistance of Cells. 



Difficulties were experienced in determining the resistance in the 

 cells which was momentarily such an inconstant quantity, owing ta 

 diffusion between the two solutions, and the difficulty was further 

 increased by polarisation when the DanielPs cell was connected. 



After conferring with Professor J. V. Jones, B.Sc., B.A., recourse 

 was had in separate experiments to the method of rapidly alternating- 

 the direction of the current sent through the cells (it being first sent 

 in the direction of the current from those cells, then reversed), and 

 reading from the first fling of the galvanometer No. 2 by the aid of 

 a reading telescope. 



The time changes in the resistance of the diffusing solutions 

 ascertained by this first fling method, are shown in the curve of 

 resistance, fig. No. 1, which is constructed from the average of a 

 series of six carefully repeated experiments, each extending over the 

 tidal period of six hours. The highest resistance of the cells at the 

 commencement was found to be 243 ohms, gradually reducing with a 

 steady curve to 12 ohms at the termination of each experiment. This 

 resistance curve, fig. No. 1, is the result of above 4,300 observations. 

 This first fling method with a Daniell's element in circuit was used 

 only for taking the rapidly changing resistance of the cells. 



General Remarks. 



A circumstance of interest in connexion with this research is 

 the change of electro-chemical position which not unfrequently 

 happened. 



Another noticeable feature was the electro-chemical position 

 maintained by the wrought iron bar (covered with its blue magnetic 

 oxide) immersed in the sea water, this bar being in the negative 

 position throughout ; repeated experiments confirmed this. 



Also when the bars were removed from the solutions at the close 

 of an experiment, the different manner in which the metals had been 

 acted upon in the respective cells was decidedly noticeable. 



From an examination of the results (taking the highest and the 

 average E.M.F.), it will be seen in what comparative manner the 

 metals arranged themselves under the conditions of the experiment. 



