The measurement of the quantity of heat energy involved in processes such as chemical reactions, changes of state, and mixing of substances, or in the determination of heat capacities of substances. The unit of energy in the International System of Units is the jaule. Another unit still being used is the calorie, defined as 4.184 joules. 
A calorimeter is an apparatus for measuring the quantity of heat energy released or  absorbed during a process. Since there are many processes that can be studied over a wide range of temperature and pressure, a large variety of calorimeters have been  developed. 
Nonisothermal calorimeters measure the temperature change that occurs during  the process. An aneroid-type nonisothermal calorimeter is normally constructed of a material having a high thermal conductivity, such as copper, so that there is rapid temperature equilibration. It is isolated from its surroundings by a high vacuum to reduce heat leaks. This type of calorimeter can be used for determining the heat capacity of materials when measurements involve low temperatures. Aneroid-type nonisothermal calorimeters have also been developed for measuring the energy of combustion for small samples of rare materials. 
With most nonisothermal calorimeters, it is necessary to relate the temperature rise to the quantity of energy released in the process. This is done by determining the calorimeter constant, which is the amount of energy required to increase the temperature of the calorimeter itself by 1°. This value can be determined by electrical calibration or by measurement on a well-defined test system. For example, in bomb calorimetry the calorimeter constant is often determined from the temperature rise which occurs when a known mass of a very pure standard sample of benzoic acid is burned. 
Isothermal calorimeters make measurements at constant temperature. The simplest example is a calorimeter containing an outer annular space filled with a liquid in equilibrium with a crystalline solid at its melting point, arranged so that any volume change will displace mercury along a capillary tube. The Bunsen ice calorimeter operates at 0°C (32°F) with a mixture of ice and water. Changes as a result of the process being studied cause the ice to melt or the water to freeze, and the consequent volume change is determined by measurement of the movement of the mercury meniscus in the capillary tube. While these calorimeters can yield accurate results, they are limited to operation at the equilibrium temperature of the two-phase system. Other types of isothermal calorimeters use the addition of electrical energy to achieve exact balance of the heat absorption that occurs during an endothermic process. 
All calorimeters consist of the calorimeter proper and a jacket or a bath, which is used to control the temperature of the calorimeter and the rate of heat leak to the environment. For temperatures not too far removed from room temperature, the jacket or bath contains liquid at a controlled temperature. For measurements at extreme temperatures, the jacket usually consists of a meta! block containing a heater to control the temperature. With nonisothermal calorimeters, where the jacket is kept at a constant temperature, there will be some heat leak to the jacket when the temperature of the calorimeter changes. lt is necessary to correct the temperature change observed to the value it would have been if there were no leak. This is achieved by measuring the temperature of the calorimeter for a time period both before and after the process and applying Newton›s law of cooling. This correction can be avoided by using the technique of adiabatic calorimetry, where the temperature of the jacket is kept equal to the temperature of the calorimeter as a change occurs. This technique requires more elaborate temperature control, and its primary use is for accurate heat capacity measurements at low temperatures. 
In calorimetric experiments it is necessary to measure temperature differences accurately; in some cases the temperature itself must be accurately known. Modem calorirneters use resistance thermometers to measure both temperatures and temperature differences, while thermocouples or thermistors are used to measure smaller temperature differences.
Heat capacities of materials and heats of combustion are processes that are routinely measured with calorimeters. Calorimeters are also used to measure the heat involved in phase changes, for example, the change from a liquid to a solid (fusion) or from a liquid to a gas (vaporization). Calorimetry has also been applied to the measurement of heats of hydrogenation of unsaturated organic compounds, the heat of dissolution of a solid in a liquid, or the heat change on mixing two liquids.