A calorimeter is any piece of apparatus in which heat is measured.
For ordinary combustions compressed oxygen is used, so that the combustible substance burns almost instantaneously, the action being induced by means of some electrical device which can be controlled from without the calorimeter.
The accuracy of heats of combustion determined in the closed calorimeter is in favourable cases about one-half per cent.
This test was applied by Joule in the well-known experiment in which he allowed a gas to expand from one vessel to another in a calorimeter without doing external work.
Under this condition the increase of intrinsic energy would be equal to the heat absorbed, and would be indicated by fall of temperature of the calorimeter.