248 Introduction to the Study of Science 



low voltage required for various uses. The tremendous pres- 

 sure at which electric current is transmitted would be impos- 

 sible without the use of the modern switch. When a circuit is 

 broken, the electric current tends to jump across the break, as 

 experience teaches. The electric current meets the resistance 

 of the air and forms an arc (page 175) of such high temperature 

 as to melt and vaporize the metal of the ordinary switch. The 

 modern switch is made to operate almost instantaneously by 

 springs, and the arc is controlled by powerful magnets. 



The uses of the electric current have multiplied rapidly in 

 recent years, with the invention of appliances for many 

 purposes, in the home, the factory, on railways, and for com- 

 munication. Lighting, heating, and cooking by electricity are 

 becoming common in many localities. In manufacturing, the 

 electric current is used in operating tools and machines of all 

 sorts, in welding, in forge and foundry work, and in electric 

 furnaces for making steel, aluminum, and nitrate (page 446). 

 The improvement of motors, together with the improved means 

 of transmission, bids fair to revolutionize transportation (pages 

 316 ff.). The progress achieved may be taken as prophetic of 

 future advance in the uses of the electric current and in the 

 utilization of falling water for the generation of electric energy 

 for all purposes. 



SUMMARY 



The use of power is one of the principal characteristics of modern 

 civilization. 



Power plants derive energy from fuel, falling water, and wind. 



The use of water power saves the coal supply. 



Falling water yields only about one fifth of the 60,000,000 horse 

 power readily available in the United States. 



The average cost of energy from falling water is about $15 per 

 horse power for a year ; from fuel about $30. 



The Big Creek-Los Angeles power plant yields power equivalent 

 to that from 7,800,000 tons of coal a year. 



Modern water wheels are highly efficient and adaptable. The Pel- 



