THE ELECTRIC CURRENT. 9 



is greatly in excess of this. The character of the chemical action 

 which takes place at an electrode also depends upon the current 

 density. Thus, copper alone is deposited upon a cathode from a 

 mixed solution of zinc and copper sulphates if the current density 

 is very small, whereas a mixture of copper and zinc is deposited 

 upon the cathode if the current density is excessive.* 



4. Faraday's lawsf of electrolysis. First law. The amount 

 of a given metal which is deposited electrolytically is propor- 

 tional to the strength J of the current and to the time, that is, 



M=klt (i) 



in which M is the amount of metal in grams deposited in / 

 seconds by a current of / amperes, and k is a constant for a 

 given metal. This constant k is called the electrochemical equiva- 

 lent ol\h.o. given metal. Electrochemical equivalents are ordinarily 

 specified in grams of metal deposited per ampere of current per 

 second. 



Second law. The electrochemical equivalents of elements 

 which can form the ions of an electrolyte, are proportional to the 

 quotients of their atomic weights divided by their valencies. A 

 metal which has two valencies has two values for its electrochem- 

 ical equivalent. Thus one and 'one half times as much iron is 



* The deposition of one metal instead of several from solutions of mixed salts 

 depends more distinctly upon the electromotive-force drop between the electrode and 

 the solution (electrode polarization) than upon current density. See Art. 22. 



f The laws of physics are the experimental facts upon which the science is based. 

 Thus Faraday's laws of electrolysis are the result of experiment, pure and simple ; 

 Boyle' s and Gay Lussac' s Laws concerning the expansion of gases are experimental facts ; 

 Newton's Laws of Motion are experimental facts ; Newton's Law of Gravitation is an 

 experimental fact ; and so on. In nearly every case the so-called laws of physics are 

 only approximately true. Thus, the product of the volume and pressure of a given 

 amount of gas at constant temperature is not strictly constant (Boyle's Law) ; the 

 amount of metal deposited by an electric current deviates in many cases from an exact 

 proportional relationship with the current (see Practical Physics, Franklin, Craw- 

 ford and MacNutt, Vol. II, page 136). 



J In Faraday's experiments, which led to the formulation of this general law, the 

 electric current was measured by a galvanometer, that is, the electric current was 

 measured in terms of its magnetic effect. 



