An instrument used to determine the heat of combustion of a substance. It typically consists of a steel chamber with a non-oxidizable material on the inside, with a tight-fitting screw lid. A known mass of sample of material is burnt in a platinum cup. Insulated platinum leads allow an electric current to be passed through a thin wire to ignite the material. The air in the chamber is replaced with pure oxygen, which is allowed to reach a pressure of around 20 atmospheres. The calorimeter is surrounded by water and once the combustion has begun, the rise in temperature recorded from which the heat of combustion is determined.