Electric Currents. 51 



light is the attendant of intense heat. But, to 

 go back to the sources of electricity. 



Frictional electric machines have been con- 

 structed in great variety. All, however, em- 

 brace the essentials set forth in the sealing- 

 wax experiment, and would be difficult to 

 describe without cuts. Let us, therefore, con- 

 sider another source of electricity, which was 

 the outgrowth of the discovery of Galvani (or 

 rather his wife), and reduced to concrete form 

 by Volta. We refer to the galvanic or voltaic 

 battery. If we put a bar of zinc into a glass 

 vessel and pour sulphuric acid and water into 

 it, there will be a boiling, and an evolution of 

 hydrogen gas, and energy is released in the 

 f . inn of heat, so that the fluid and the glass ves- 

 sel become heated. Now let us put a bar of 

 copper or a stick of carbon into the glass, but 

 not in contact with tho zinc; connect the ends 

 (that are not immersed) of the two elements 

 copper and zino with a metal wire or any con- 

 ductor, and a new condition is set up. Heat is 

 7ii 1 Mirror evolved to the same extent, but most 

 of tli becomes electrical in character, 



;m<1 an elect riral chain of action takes place in 

 the circuit that has now been formed. Taking 

 the zinc MS the starting point, tho so-csillrd 

 current flows from the /inc through tho fluid 

 to thr copper and from tho copper through the 

 wire to the zinc. 



A chain of polarized atomic activity is es- 



