THE STEAM ENGINE 339 



steam engine in the generation of electric power, or in 

 the transmission of any other form of energy derived from 

 steam. 



The working of the De Laval turbine is as follows : The stearn is 

 blown through stationary divergent nozzles where it is allowed to 

 expand to the pressure of the exhaust chamber. Each particle of 

 steam, which moves very rapidly, strikes against a concave vane or 

 plate which projects from the drum like a spoke. This causes the 

 wheel to move rapidly. The outer end of the buckets are covered 

 by a ring which prevents the centrifugal escape of the steam. The 

 nozzles vary in number and can be closed independently of each 

 other, so that the number in use may be made to suit conditions of 

 running. 



As the material composing the turbine machine limits the speed at 

 which it can safely be run, it is necessary to have some form of re- 

 ducing gear in the transmission. The smaller types of De Laval 

 turbines run at about 30,000 R. P. M., and are geared down to about 

 3000. The larger sizes run at about 10,000 R. P. M. under gear. 

 Even with all the disadvantages of gearing, the turbine is used ex- 

 tensively in units ranging from 1^ to 200 H. P. 



Its principal parts are the shaft, drum, cylindrical case inside of 

 which the drum revolves, vanes on the drum and cylindrical part, 

 balance pistons. 



381. Measurement of Work in Heat Units. Experiments 

 show that one unit of heat is equivalent to 772 ft.-lbs. of 

 work, and when this quantity of work disappears in friction, 

 one unit of heat is generated. Other experiments show that 

 the unit should be 778 ft.-lbs. It is not of much importance 

 which number is used ; some use one, and some use the other, 

 but all agree in naming this quantity of work after the dis- 

 coverer of the relationship, James P. Joule of Manchester, 

 England. The unit is therefore called Joule's equivalent, or 

 the mechanical equivalent of heat. 



