306 MEASUREMENT OF CURRENTS. 



As positive electrode, we may use either a platinum plate or a 

 plate of the same kind as the metal deposited, and which, as it 

 dissolves, reforms the salt decomposed by electrolysis. With a 

 platinum plate the solution becomes poorer, and acid is set free ; 

 but this does not affect the deposit provided the reduction of the 

 salt is not carried to the point of materially altering the strength 

 of the solution. 



When two plates of the same metal are used, the soluble 

 electrode should theoretically lose all that the other has gained ; 

 and it should therefore be a matter of indifference which plate is 

 weighed. This is found to be the case with plates of pure silver 

 in a 15 per cent, solution of nitrate of silver and a suitable density 

 of the current. The alteration in weight is exactly the same in 

 the two plates, and this agreement is an excellent control of the 

 weighings. But general experience shows that, for "copper more 

 particularly, the loss on the positive plate is greater than the gain 

 of the negative one. The metal, in becoming disaggregated, falls 

 in particles of extreme minuteness in the liquid, and special pre- 

 cautions must be taken that the particles are not deposited on 

 the negative plate. There are found, moreover, on the soluble 

 plate oxygen compounds, the importance and nature of which de- 

 pend on the intensity of the current.* 



The most recent experiments have given for the chemical action 

 of an ampere, 



Per Second. 



Silver. Water. 



Mg. Mg. 



Kohlrausch K '*4&3 '93 2 5 



Mascart 1-1156 0*09303 



Lord Rayleigh i'H79 0*09321 



From the mean of these numbers the weight of silver deposited 

 by an ampere in an hour is 



4 '02 2 grammes. 



* See F. and W. KOHLRAUSCH. Site, des Phys. Med. Ges. zuWUrzburg, 1884. 

 RAYLEIGH. Phil. Trans., Part II., p. 411. 1884. MASCART. Journal de Phy- 

 sique [2], Vol. I., p. 109. 1882. And Vol. in., p. 283. 1884. 



