A Beckmann thermometer is a device used to measure small differences of temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, but not absolute temperature values. It was invented by Ernst Otto Beckmann

Ernst Otto Beckmann

Ernst Otto Beckmann was a German chemist who is remembered for his invention of the Beckmann differential thermometer and for his discovery of the Beckmann rearrangement.-Scientific work:...

 (1853 – 1923), a German chemist

Chemist

A chemist is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties such as density and acidity. Chemists carefully describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms...

, for his measurements of colligative properties

Colligative properties

Colligative properties are properties of solutions that depend on the number of molecules in a given volume of solvent and not on the properties/identity of the molecules. Colligative properties include: relative lowering of vapor pressure; elevation of boiling point; depression of freezing point...

 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 Today its use has largely been superseded by electronic thermometers.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.

Calibration

In setting the thermometer, a sufficient amount of mercury must be left in the bulb and stem to give readings between the required temperatures. First, the thermometer is inverted and gently tapped so that the mercury in the reservoir lodges in the bend (B) at the end of the stem. Next, the bulb is heated until the mercury in the stem joins the mercury in the reservoir. The thermometer is then placed in a bath one or two degrees above the upper limit of temperatures to be measured.

The upper end of the tube is gently tapped with the finger, and the mercury suspended in the upper part of the reservoir will be jarred down, thus separating it from the thread at the bend (B). The thermometer will then be set for readings between the required temperatures.

Further details and cautions are given in books of the "External links" section below.