MEASUREMENTS OF CURRENTS BY ELECTROLYSIS. 305 



918. MEASUREMENTS OF CURRENTS BY ELECTROLYSIS. Elec- 

 trolysis furnishes another means of measuring currents which may 

 be useful in many cases for instance, in the direct valuation of 

 powerful currents. 



It follows from Faraday's laws, that the chemical action of the 

 current, is proportional to the quantity of electricity which passes 

 (955). The ratio of the quantity of electricity decomposed, to 

 the corresponding time, will give the intensity of the current if 

 this is constant, or, in the opposite case, its mean intensity. The 

 bodies which have been most used, and which, in fact, seem to 

 suit best, are acidulated water, a solution of copper sulphate, or 

 a solution of nitrate or chlorate of silver. 



With water the volume of gas liberated is measured. The gases 

 should be dried and their temperature and pressure known. In 

 order to avoid these corrections, Bunsen finds it better to weigh 

 the water decomposed. The voltameter is weighed before the 

 experiment, and the gases evolved pass through a drying tube 

 which absorbs the aqueous vapour, and is then again weighed 

 after displacing the residual gases by air. One cause of error 

 arises from the quantity of gas dissolved in water. This might 

 be removed by exhausting the gas with a mercury pump. 



The water should be acidulated. When sulphuric acid is used, 

 ozone, peroxide of hydrogen, and other secondary products are 

 formed, and the volume of gas collected is too small. If, how- 

 ever, the temperature of the voltameter is raised to 50, these 

 oxygenated compounds are formed in negligable quantities. 



Experiments are more convenient and more certain with metal 

 salts. The essential condition is to obtain on the negative electrode 

 a coherent deposit which can be easily washed, so as to get rid 

 of all trace of acid, and which does not oxidize in the air. 



The form of the deposit depends more particularly on the density 

 of the current that is, the quotient of the intensity by the surface 

 of the electrodes. With copper the deposit is only good when 

 this density is small. If it increases it is rough and botryoidal. 

 With a stronger current it becomes pulverulent. The concentration 

 of the solution has a much smaller influence. The plate covered 

 with copper should be placed for a few moments in boiling distilled 

 water immediately on being taken out of the bath, then wiped and 

 dried with blotting-paper. It would rapidly oxidize if left while 

 moist in the air. 



With silver the plate, when once washed with distilled water, 

 may be left to dry spontaneously. 



VOL. II. X 



