A Beckmann thermometer is a device used to measure small differences of temperature, but not absolute temperature values. It was invented by Ernst Otto Beckmann (1853 - 1923), a German chemist, for his measurements of colligative properties in 1905. Today its use has largely been superseded by electronic thermometers.
A Beckmann thermometer's length is usually 40 - 50 cm. The temperature scale typically covers about 5 °C and it is divided into hundredths of a degree. With a magnifier it is possible to estimate temperature changes to 0.001 °C. The peculiarity of Beckmann's thermometer design is a reservoir (R on diagram) at the upper end of the tube, by means of which the quantity of mercury in the bulb can be increased or diminished so that the instrument can be set to measure temperature differences at either high or low temperature values. In contrast, the range of a typical mercury-in-glass thermometer is fixed, being set by the calibration marks etched on the glass or the marks on the printed scale.