Wedgwood scale

The Wedgwood scale is a temperature scale for higher temperatures, such as those found in porcelain or metal production. It is named after the British entrepreneur Josiah Wedgwood , who proposed it in 1782.

scale

Wedgwood stated that the zero point of its scale is converted to 581 ° C. This is the temperature at which he first noticed the faint red heat. Each degree of Wedgwood should then correspond to a temperature increase of 72 ° C. Wedgwood also gives some melting points of metals as comparative values, copper 27 °, silver 28 °, gold 32 ° and bronze 21 °. Guyton de Morveau later corrected both the starting point and the conversion of the degree steps. At de Morveau, the starting point is 270 ° and each degree of Wedgwood is only 34 ° C.

method

Wedgwood used the decrease in volume of clay during firing for its measuring method . He used small clay cylinders 0.5 inch in diameter , always of the same origin and composition. These clay cylinders were then exposed to the environment to be measured and, after cooling, the decrease in diameter was determined. However, since the decrease is only small, it is difficult to determine precisely. Wedgwood also developed a simple device to easily read the temperature, this is often called a Wedgwood pyrometer. Two metal strips were attached to a metal sheet in a slightly V-shape. At the wider end, the gap is 0.5 inches, which is just enough for an unfired clay cylinder, while at the lower end the gap is only 0.3 inches. 240 tick marks are now entered over the entire length. Clay cylinders to be measured are now allowed to slide into the converging rails and read from where the cylinder is stuck.

history

Measurement with a mercury thermometer is only possible below the boiling point of mercury, i.e. 350 ° C. For a long time it was not possible to determine higher temperatures. The porcelain manufacturer Josiah Wedgwood now developed a measuring method for determining the firing temperature and published the method in 1782. The method was used by many scientists and also in industrial technology, although the incorrect comparison with the other temperature scales gave the scale completely excessive values. With the invention of the platinum pyrometer by John Frederic Daniell around 1830 and the use of thermocouples for temperature measurement, the method was no longer used.

Sometimes the sailing cones are still used for temperature control. However, these do not change when the clay sintered together, but rather they collapse due to partial melting.

literature

Justus Liebig: Concise Dictionary of Pure and Applied Chemistry, Volume 6 (pp. 713-714) . Vieweg and Son, 1854 ( full text in the Google book search).

Overview of the classic temperature scales

unit	Unit symbol	lower anchor point F 1	upper anchor point F 2	Unit value	inventor	Year of creation	Distribution area
Kelvin	K	Absolute zero point , T 0 = 0 K	Now without a fixed point, originally later T Tri ( H 2 O ) = 273.16 K ${\displaystyle 1\,\mathrm {K} \;{\stackrel {\mathrm {def} }{=}}\;1\,^{\circ }\mathrm {C} }$	${\displaystyle 1\,\mathrm {K} =1{,}380\,649\cdot 10^{-23}{\frac {\mathrm {J} }{k_{\mathrm {B} }}}}$ earlier ${\displaystyle {\frac {F_{2}-F_{1}}{273{,}16}}}$	William Thomson Baron Kelvin	1848	worldwide ( SI unit )
centigrade	° C	Now 0 ° C = 273.15 K, previously T Schm (H 2 O) = 0 ° C	Now coupling to Kelvin, previously T boiling (H 2 O) = 100 ° C	${\displaystyle 1\,^{\circ }\mathrm {C} \;{\stackrel {\mathrm {def} }{=}}\;1\,\mathrm {K} }$ earlier ${\displaystyle {\frac {F_{2}-F_{1}}{100}}}$	Different Celsius	1742	worldwide ( derived SI unit )
degrees Fahrenheit	° F	Now 32 ° F = 273.15 K, originally T cold. = 0 ° F, later T Schm (H 2 O) = 32 ° F	Now coupling to Kelvin, originally T human = 96 ° F, later T boiling (H 2 O) = 212 ° F	${\displaystyle 1{,}80\,^{\circ }\mathrm {F} \;{\stackrel {\mathrm {def} }{=}}\;1\,\mathrm {K} }$ originally later ${\displaystyle {\frac {F_{2}-F_{1}}{96}}}$ ${\displaystyle {\frac {F_{2}-F_{1}}{180}}}$	Daniel Fahrenheit	1714	United States
Rankine degree	° Ra, ° R	T 0 = 0 ° Ra	Now coupling to Kelvin	${\displaystyle 1\,^{\circ }\mathrm {Ra} \;{\stackrel {\mathrm {def} }{=}}\;1\,^{\circ }\mathrm {F} }$	William Rankine	1859	United States
Degree Delisle	° De, ° D	T Schm (H 2 O) = 150 ° De	T boiling (H 2 O) = 0 ° De	${\displaystyle {\frac {F_{1}-F_{2}}{150}}}$	Joseph-Nicolas Delisle	1732	Russia (19th century)
Degree Réaumur	° Ré, ° Re, ° R	T Schm (H 2 O) = 0 ° Ré	T boiling (H 2 O) = 80 ° Ré	${\displaystyle {\frac {F_{2}-F_{1}}{80}}}$	René-Antoine Ferchault de Réaumur	1730	Western Europe until the end of the 19th century
Degrees Newtons	° N	T Schm (H 2 O) = 0 ° N	T boiling (H 2 O) = 33 ° N	${\displaystyle {\frac {F_{2}-F_{1}}{33}}}$	Isaac Newton	≈ 1700	none
Degree Rømer	° Rø	T Schm ( Lake ) = 0 ° Rø	T boiling (H 2 O) = 60 ° Rø	${\displaystyle {\frac {F_{2}-F_{1}}{60}}}$	Ole Romer	1701	none
Notes on the table: ↑ T Tri (H 2 O) has been at 273.16 K since the redefinition in May 2019 with a relative uncertainty of 3.7 · 10 −7 according to Le Système international d'unités . 9e édition, 2019 (the so-called "SI brochure", French and English), pp. 21 and 133. ↑ The temperature of a cold mixture of ice, water and salmiak or sea salt (−17.8 ° C) and the supposed "body temperature of a healthy person" (35.6 ° C) were originally used ↑ The melting temperature of a brine (−14.3 ° C) was used.