In 1804, Benjamin Thompson, or: Count Rumford, described experiments on the heat radiation of various substances. For this, he filled two cylindrical heat radiators with water of a certain temperature (usually boiling) and placed them on either side of a thermoscope, a kind of differential thermometer.

The radiators were made of brass or lead (tinned steel), and the circular part of one of the surfaces was sometimes coated with other materials such as gold or silver leaf and polished or smutted. The thermoscope consisted of two blackened glass spheres connected by a capillary. A drop of coloured ethanol was placed in the centre of the capillary. When the temperature of the air inside one of the two glass spheres exceeds that of the other, the air inside expands, causing the drop of liquid inside the capillary to move.

The two radiators and the thermoscope were placed on a wooden box, which allowed the distance between the radiators and the thermoscope to be varied. Between the two spheres of the thermoscope was a circular screen made of copper, through which each radiator was to act only on the sphere facing it. To shield the body heat of the person conducting the experiment, two copper shields were placed between the person and the rest of the set-up.