An electric current in a wire will always generate heat. In this instrument that heat is transferred to the air in a glass bulb. The air will expand, which is possible only at the bottom side, where a narrow, downwards sloping tube leads to a reservoir filled with mercury. As the air in the bulb gets warmer, the mercury level in the tube will descend. The height can be read on a scale in tenths of a French inch. By making the slope of the tube less steep, the descend of the mercury level can be better observed. This apparatus is an adaptation by Peter Theophil Riess (1804-1883) of the thermometer of Snow Harris (see cat. no. 731 / FK 560).
Electric thermometer, after Snow Harris and Riess
pre 1882