208 MR THOMAS ANDREWS ON THE RELATIVE 



was a low resistance galvanometer, with jewelled centre, accurately graduated 

 throughout the circle. 



To observe the fractional parts of degrees with accuracy, the author took 

 the observations through a powerful lens fixed above the galvanometer, by 

 means of which arrangement and from the author's experience in using it in 

 this manner, and from repeated check experiments, it was found that variations 

 of the needle could be taken to about the -f^th of a degree of deflection. 



The galvanometer was examined by the Wheatstone Bridge arrangement 

 with standard resistance coils, and the resistance was found to be 220 ohms. 



A Daniells cell through a resistance of 9180 ohms (including the resistance 

 of the galvanometer) gave a deflection of one degree, or ^ T Voth of an ampere 

 produces a deflection of one degree (taking the electromotive force of the 

 Daniells cell as unity). 



From these observations, therefore, the strength of the electric current 

 represented by the deflections recorded in this paper may be calculated ; and 

 as the resistance in the cells containing the sea- water would not exceed 

 3 ohms, an indication of the E.M.F. can be obtained. 



In taking all readings the galvanometer was very carefully adjusted before 

 each observation, and all deviations from vibration, tremor, or other causes, 

 carefully guarded against; the greatest care was exercised in the experiments, 

 the point of the needle being under constant observation, and the slightest 

 variations were carefully watched as the differences to be dealt with in course 

 of these experiments were sometimes small. 



The author is satisfied that the observations recorded represent accurately 

 differences arising from the nature of the metals and solutions employed.] 



The experiments on bars, recorded in tables D, E, F, represent the average 

 of the deflections observed on first immersion of the steel and iron bars in the 

 sea-water, the bars on each repetition of the experiment being carefully washed 

 and wiped dry before re-immersion. 



A very marked feature in the other galvanometer experiments, recorded in- 

 table and diagram G, I, Plate XXXII. (these observations extending over 

 longer periods of time), is the steadily changing deflections noticed from the 

 commencement; this appears to indicate a tendency in the various steels and 

 irons to polarise each other's electric action, so that in course of time, as 

 submerged iron and steel becomes coated with oxides, galvanic activity is con- 

 siderably reduced from its first force; but it does not wholly subside. When 

 plates of iron and steel, &c, in galvanic connection were taken out of the sea- 

 water (the oxides being washed off) and were re-immersed, the deflections of 

 the galvanometer arose for a time, afterwards reducing Again. 



The experiments in this memoir indicate that the galvanic relationships of 

 the various steels and wrought iron do not remain the same, in sea-water or 

 Other solutions, but they appear capable of an interchange of electro-chemical 



