CHAPTER XI. 



DYNAMICAL ELECTRICITY. 



ABOUT the year 1786, Luigi Galvani, a professor of anatomy, 

 discovered that in the case of a recently killed frog, if one end of 

 a metal conductor, composed of zinc or copper, was placed in 

 contact with the nerves of the lumbar region and the other with 

 muscles of the thigh or leg, a prompt contraction of the muscles 

 ensued. He assumed that the electricity was inherent in the 

 animal substance; but Professor Volta from his experiments, 

 attributed to the metals the active part of the phenomena of 

 contraction. The investigations of these men and their suc- 

 cessors have led up to the dynamical electricity of to-day. 



A little experiment will help us to understand how this form of 

 electricity is developed. Get a strip of sheet copper and two pieces 

 of zinc, each about five inches long and one and one-half inches 

 wide; and have a piece of No. 16 copper wire about ten inches 

 long soldered to one end of each. Fill a tumbler about two-thirds 

 full of water and add to it two tablespoonfuls of strong sulphuric 

 acid. Amalgamate one piece of zinc as follows: Dip it into the 

 acidulated water, and then pour a little mercury over the surface, 

 rubbing off any excess with a cloth. Put a piece of copper and a 

 piece of zinc that is not amalgamated into the liquid without 

 allowing them to touch each other, and vigorous action takes 

 place between the acid and the zinc, but none between it and 

 the copper. Now substitute amalgamated zinc for the other 

 and no action appears on either plate. Now, if the wires of the 

 two plates are brought in contact, gas is given off from the cop- 

 per but not from the zinc plate; and, if we separate the wires care- 

 fully, we may see a little spark of light, and the action in the fluid 

 ceases. There is no action between the liquid and the amalga- 

 mated zinc, unless it is connected with the copper. The wire 



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