8 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



present define the strength of an electric current as proportional 

 to the amount of a given metal deposited by the current per second 

 in an electrolytic cell. 



The international standard ampere is defined * as that strength 

 of current which will deposit o.ooi 1 18 gram of silver per second 

 from an aqueous solution of pure silver nitrate. Another unit of 

 current, the abampere or c.g.s. unit, is defined in Art. 52. 



The coulombmeter. An electrolytic cell arranged for the meas- 

 urement of current by weighing the amount of metal deposited by 

 the current in a given time is called a coulombmeter.f Thus, 

 the copper coulombmeter consists of a glass vessel containing an 

 aqueous solution of copper sulphate and having sheet-copper 

 electrodes. The cathode, or gain-plate, is weighed at the begin- 

 ning and again at the end of the run, and the strength of the cur- 

 rent is calculated by dividing the observed amount of deposited 

 copper by the amount of copper that would be deposited in the 

 same time by one ampere. 



Current density at an electrode. The quotient of the current 

 flowing through an electrolytic cell divided by the active area of 

 one of the electrodes is called the current density at that electrode. 

 The physical character of the metal which is deposited by an 

 electric current depends very greatly upon the current density at 

 the electrode upon which the metal is deposited. Thus, metallic 

 copper is deposited from a solution of copper sulphate as a 

 smooth, solid layer if the current density does not exceed 0.02 

 ampere per square centimeter, whereas the deposit becomes veiy 

 rough with projecting crystals of the metal if the current density 



* In accordance with the recommendations of the International Electrical Congress 

 which met at Chicago in 1893. The fundamental definition of the ampere is based 

 upon the magnetic effect of the electric current as explained in Art. 52. The value 

 of a current in amperes (as defined by the magnetic effect) may be determined from 

 purely mechanical measurements as explained in Art. 59. In this way the amount of 

 silver deposited in one second by one ampere may be determined. This determination 

 has been made a number of times with great care, the latest determination being that 

 of H. S. Carhart and G. W. Patterson. See Journal of the Institution of Electrical 

 Engineers, Vol. 34, pages 185-189, February, 1905. 



f Sometimes called a voltameter. 



