A machine is a combination of rigid members which have definite motions and which are capable of performing useful work. There are two types of machines : power engines which convert natural or electrical energy into mechanical energy and working machines which are driven by power engines and produce useful work. Natural energies are the energy of water dammed up in rivers or stored in natural or artificial lakes, the chemical energy of fuels and atomic energy. Electrical energy is a transformed energy which is used for the transport of energy. There are the following power engines: reciprocating steam engine, internal combustion engine, water-, steam-, and gas turbine and the electric motor. Examples of working machines are: pumps and compressors, machine tools, concrete mixers, elevators etc. and the electric generator.
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