390 TENTH ANNUAL REPORT OF 



To reduce the deflection of the compass-needle from 20° to 10° the 



following must be inserted : 



With metallic closing 19.91 rheostat coils. 



A decomposing cell being inserted in the 

 closing arc, formed of two plates of plati- 

 num immersed in nitric acid 17.37 " 



Polarization of the decomposing cells ... 2.54 



By this method the following values were found for the galvanic 

 polarization of different decomposing cells : 



Copper-plates in sulphate of copper 0.07 



Amalgamated zinc plates in nitric acid 0.03 



Copper-plates in nitric acid 0.01 



These experiments prove that polarization disappears when the 

 escape of gas ceases at the electrodes ; in all three cases no oxygen ap- 

 peared at the positive electrode, because it oxidized the metal imme- 

 diately on its evolution from tlie water ; the escape of hydrogen at the 

 negative electrode was prevented in the first case by attracting in its 

 nascent state the oxygen from the oxide of copper, and precipitating 

 metallic copper ; in the other two cases the nascent hydrogen was 

 immediately oxidized by the nitric acid. 



Thus here, where the electrodes are not covered with a stratum of 

 gas, polarization does not take place ; the small numerical values 

 given above are not due to the polarization of the electrodes, but to 

 the fact that they do not remain in the same state — one plate being 

 attacked and the other not, and thus the pair of plates itself becomes 

 a feeble electro-motor. 



Buff also (P. A., LXXIII, 497) found the polarization for copper 

 plates in sulphate of copper, and for zinc plates in sulphate of zinc, 

 very small. 



Lenz and Saweljev found further for the polarization of 



Platinum plates in nitric acid 2.48 



Platinum plates in sulphuric acid* 5.46 



Amalgamated zinc plates in SO3 1.00 



Copper plates in " ." 2.15 



Tin electrodes " 1.45 



Iron electrodes " 0.33 



Graphite in concentrated " 1,26 



These numerical values are mostly the mean results of a number of 

 experiments. 



In the first case, that of platinum plates in nitric acid, there is no 

 escape of hydrogen at the negative electrode — the polarization shown 

 in the value 2.48 is thus to be ascribed entirely to that at the positive 

 electrode, where oxygen appears ; 2.48 is consequently the magnitude 

 of the polarization which a platinum plate receives from oxygen: 



In the second case^ that of platinum plates in sulphuric acid, de- 

 velopment of gas takes place at both electrodes ; therefore 5.46 is the 



* Composed of 6 vols, of concentrated SO^ -f- 100 of water. 



