THE PRINCIPLES OF ENERGY 39 



ever form this capacity for doing work may exist, it can always 

 be measured, directly or indirectly, as a certain quantity of 

 mechanical work performed. That means that we do not know 

 what available energy is, we only know what it does. 



Energy Transformations. 



We may profitably expand the result stated in the last para- 

 graph, and give some further examples. There is a heap of coal, 

 and this we shall call available chemical energy. It contains 

 certain substances carbon and hydrocarbons and these can 

 combine chemically with the oxygen of the air to form carbonic 

 acid gas and water (with some other substances). In this act of 

 chemical combination heat is generated. If the burning fuel is 

 confined in a furnace, a large fraction of the heat may be commu- 

 nicated to the water contained in a steam boiler, and its effect is 

 to increase the motions of the molecules of the water. By-and- 

 by these motions become so rapid that the molecules fly apart 

 and the water is converted into steam. Later on the velocities of 

 the molecules of the steam are increased by the further flow of 

 heat from the furnace, and they collide with, and rebound from, 

 the walls of the boiler in which they are contained, thus setting 

 up steam pressure. 



The chemical energy of the fuel is therefore transformed 

 into the kinetic energy of the molecules of steam under 

 pressure. 



The steam is then allowed to expand into the cylinders of the 

 engine, and in so doing it pushes out the pistons and communi- 

 cates a rotatory motion to the crank shafts. 



Thus the kinetic energy of the molecules of steam is trans- 

 formed into the kinetic energy of the large, moving parts 

 of the engine that is, into mechanical energy. 



The motion of the engine is next communicated, by means of 

 shafting or belts, to the armature of a dynamo. When the latter 

 revolves it generates an electric current, and this uses up the 

 mechanical energy; for if the switch of the dynamo is open, 

 relatively little power is required to rotate the latter, and no 

 current -passes. But when the switch is closed, relatively much 

 power is required, and a current is generated. 



Mechanical energy therefore transforms into electric energy. 



