86 INTRODUCTION TO GENERAL SCIENCE 



c. 1305:108. The Aurora. 



d. 1309 : 277-278. The Aurora and its Cause. 



e. 1312 : 396-397. The Aurora and its Cause. 

 /. 1801:336. The Aurora. 



64. SOURCES OF ELECTRICITY 



There are three sources of electricity frictional, chemi- 

 cal, and magnetic. The quantity of electricity produced by 

 friction is very small. Whenever one substance is rubbed by 

 another, electricity results and may be easily detected. Two 

 pieces of lump sugar, if rubbed together in a dark place, will 

 give out slight flashes of light. It is possible to obtain an 

 electric spark from a cat, by rubbing its back and then touch- 

 ing its nose or ear. Put the cat on an insulated chair, that 

 is, have plates of glass under the legs of the chair. 



Whenever two dissimilar elements are placed in a suitable 

 solution, and their ends joined, a current of electricity passes 

 which varies in strength according to the elements and the 

 solution. Such a combination is called a galvanic cell, and 

 modifications of these cells are used largely for electric bells 

 and gas ignition. 



By far the greatest amount of electricity is produced by 

 moving a wire near a magnet. If such a wire has its ends 

 joined, a current of electricity will pass through it. The cause 

 for this production of electricity is not known. The dynamo 

 is the practical application of this principle. 



References : 



1. 1103:175-179. Atmospheric Electricity. 



2. 1803 : 238-239. Development of Electrification by Friction. 



3. 1803 : 262-263. The Galvanic Cell. 



4. 1803 : 282-283. The Theory of the Simple Cell. 



