A calorimeter is an apparatus for measuring heat quantities generated in or emitted by materials in processes such as chemical reactions, changes of state, or formation of solutions.
The French scientists Antoine-Laurent de Lavoisier and Pierre-Simon Laplace invented the calorimeter in 1780 to research the amount of heat released or absorbed by various substances and chemical reactions. Here is a diagram of the exterior of the calorimeter they constructed:
Here is a diagrammatic section of the same calorimeter:
The external compartment is meant to insulate the interior one from heat from the outside of the calorimeter. As the object inside the metal basket releases heat, it melts the ice in the interior compartment and as a result water flows through the tap into the container below. A thermometer was placed on the lid of the calorimeter to measure the temperature of the object in the basket. Lavoisier and Laplace calculated the heat released by the objects based on their temperatures and how much water was collected in the container.
Here is a portrait of Antoine-Laurent de Lavoisier: