214 



SIMPLE ELECTRIC DEVICES 



If we wish to avoid loss of current by heating, we use a 

 wire that offers little resistance. If we wish to transform elec- 

 tricity into heat, as in the electric stove, we choose wire of 

 high resistance, as German silver wire. 



Chemical effects. The plating of gold, silver, and other 

 metals. If strips of lead or rods of carbon are connected to 

 the terminals of an electric cell, as in Figure 82, and are then 

 dipped into a solution of copper sulphate, the strip in connec- 

 tion with the negative terminal of the cell soon becomes thinly 

 plated with a coating of copper. If a solution of silver nitrate 



is used in place of the copper sul- 

 phate, the coating is of silver in- 

 stead of copper. As long as the 

 current flows and there is any 

 metal present in the solution, the 

 coating continues to form on the 

 negative electrode, and becomes 

 thicker and thicker. 



The process by which metal is 

 taken out of solution, as silver 

 out of silver nitrate and copper out of copper sulphate, and is 

 in turn deposited as a coating on another substance, is called 

 electroplating. An electric current separates a liquid into some 

 of its various constituents and deposits one of the metal constit- 

 uents on the negative electrode. 



Since copper is constantly taken out of the solution of copper 

 sulphate for deposit upon the negative electrode, the amount of 

 copper remaining in the solution steadily decreases, and finally 

 there is none of it left for deposit. In order to overcome this, 

 the positive electrode is made of the same metal as that which is 

 to be deposited. The positive metal electrode gradually dissolves 

 and replaces the metal lost from the solution by deposit, and elec- 

 troplating can continue as long as any positive electrode remains. 



FIG. 82. Carbon rods in a solution 

 of copper sulphate. 



