SCIENCE AND PRACTICE. 279 



To arrive at e y we must eliminate E, in order to do which 

 two other observations are necessary. First, the constant 

 cell, E, is connected in the circuit of the galvanometer, and 

 a resistance, =W, is added by degrees until the needle 

 indicates again the intensity I, whence 



-*' 



and still more resistance, p, until the intensity is further 

 reduced to I,, the same as in equation (II., by which 



From these two equations the value of E is obtained 



n - 



which, subtracted from (III., leaves the value of e 



The polarisation is, therefore, equal to the product of the 

 two indications of the galvanometer divided by their dif- 

 ference, and multiplied by the difference of the resistances 

 added to the circuits, to reduce the deflections each time from 

 I to I P 



Lenz and Saweljev found the polarisation of plates of 

 different metals in different fluids to be as follows : 



1. Platinum plates in diluted sulphuric acid (6 parts 



acid to 100 parts water) 1185 



2. Platinum plates in nitric acid 538 



3. Copper plates in sulphuric acid 466 



4. Zinc plates in sulphuric acid 315 



5. Graphite plates in nitric acid 273 



6. Amalgamated zinc plates in sulphuric acid 217 



7. Iron plates in sulphuric acid 72 



When, however, the fluids are of such a nature that no gas 

 is formed on either of them, that is to say, when the gases 

 are recombined in the moment of their formation, little or no 

 polarisation is observed. 



