1 68 GENERAL SCIENCE 



pended upon it equals the moment of the resistance offered to its 

 motion. 



In the case of levers the ratio between the distances through which 

 power and weight move is just the same as that of their lever arms 

 which are radii of the arcs described. 



ELECTROMAGNETS 



So common is the use of electricity in modern life that in 

 one form or another its phenomena are familiar to all. 

 The uses of electricity are so important that some knowledge 

 of its development within recent years becomes necessary. 

 It is not complimentary to one's understanding of the times 

 in which he lives, and of the conditions affecting the affairs 

 of his day, not to possess this knowledge. Because of the 

 widespread use of electricity, and its ever growing importance 

 in the affairs of men, this may be called an " electrical age". 



The scientist sees in the water-fall of some mountain stream 

 a* vast amount of stored-up solar energy previously expended 

 on the water in its evaporation, and in its transportation 

 to the higher altitudes where condensation took place. 

 The man of affairs in the world sees the possibility of con- 

 verting into electrical energy the power for doing work 

 possessed by this running water, and of transferring this 

 energy over wires to distant towns and cities where it may 

 contribute to man's comfort and serve him in manifold ways. 



A coil of wire of many turns, and whose ends are joined 

 to make a "closed circuit," has an electric " current" de- 

 veloped in it when it is made to rotate very rapidly within 

 the influence of a powerful magnet. The discovery of this 

 fact by the English scientist Faraday in I83I 1 was a wonder- 

 ful step forward in the mastery of man over the forces of 



1 And by Joseph Henry, an American, at a somewhat earlier date. How- 

 ever, it was about forty years later before the dynamo became an industrial 

 reality. 



