The steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.
The idea of using boiling water to produce mechanical motion has a long history, going back about 2,000 years. Early devices were not practical power producers, but more advanced designs (by people such as Savery, Newcommen, Watt and Trevithick) producing usable power have become a major source of mechanical power over the last 300 years.
Relation to Power:
One of the main demands for coal was to power steam engines; although much of the early Industrial Revolution was achieved without mechanisation, or using water power, steam power when it became practical allowed factories to be built away from water, thus allowing for considerably more of the country to become industrialised.
However this further accelerated the movement of people into cities to take up semi-skilled labour, and other results of industrial growth on British society. This was already causing protests and riots, and things would only get worse.