208 Our Surroundings 



generally accepted. It assumes that all atoms are made tip of 

 minute particles of electricity, both positive and negative. If you 

 could observe an atom, you would see the positive particles, called 

 protons, forming the center, or nucleus, about which the negative 

 particles, called electrons, revolve much as the planets revolve 

 around the sun. 



Here is a simple illustration of this theory. If we rub a glass 

 tumbler with a piece of silk, the silk is supposed to rub electrons 

 from the glass, leaving more positive electricity than negative. 

 The glass is thus left positively charged. The silk, having picked 

 up negative electrons from the glass, has become negatively 

 charged. This is like rubbing one's wet hands with a towel. The 

 towel picks up moisture from the hands and becomes wet while 

 the hands become dry. 



Current Electricity. It has been stated that electricity, like 

 water and gas, will flow and exert pressure under suitable con- 

 ditions. When it flows it is called current electricity. To be 

 of practical use the current must always be flowing. To accom- 

 plish this there must be a difference in potential, or amount of 

 electrical charge, which must constantly be maintained. How to 

 maintain it is the problem. 



The Electric Cell. The electric cell is the result of the 

 discovery by two Italians, Yolta and Galvani, that electric charges 

 can be produced by chemical action. This is why the electric 

 cell is sometimes referred to as the Voltaic cell, and sometimes 

 as the Galvanic cell. Volta experimented with a cell consisting 

 of a strip of zinc and a strip of copper placed opposite each other, 

 but not in contact, in a glass of water to which a few drops of 

 acid had been added. He found out that when the tops of the 

 zinc and copper strips were joined with a wire, there was a con- 

 tinuous flow of electricity through the wire from the copper to 

 the zinc. Chemical energy was thus changed to electrical energy. 



The Circuit. The circuit is the course the current takes 

 in its complete passage from the starting point back to the same 

 point. Not only must there be an unbroken path through the wire 

 between the copper and the zinc plates, but there must also 

 be a similar path through the liquid that separates the plates; 



