Royal Holloway College McGill University Hugh Longbourne Callendar FRS (18 April 1863 – 21 January 1930) was a British physicist known for his contributions to the areas of thermometry and thermodynamics.

[2] Callendar worked with multiple institutions during World War I, helping to research and develop useful tools for the Navy.

[4] They had a daughter, Cecil (1895), and three sons, Guy Stewart Callendar (1898), Leslie Hugh (1896) and Maxwell Victor (1905).

He published a number of works on what he called "Cursive Shorthand"[8][9] and wrote on English spelling reform.

Prior to Callendar’s work in the area of thermometry, there lacked an instrument for accurate and reliable temperature measurement.

[10] At Cavendish Laboratory, Thomson advised Callendar to study metallic resistance thermometry.

[4] Werner von Siemens was the first to propose the use of a platinum resistance temperature detector in 1860, although his instrument readings were unstable.

[2][11] By 1886 he had developed a design for an accurate platinum resistance thermometer, correcting the errors made by Siemens.

His results were praised by J.J Thomson for providing a new tool that “could determine temperatures with an ease and accuracy never obtainable before”.

[4][10] His design underwent vigorous testing at the National Physical Laboratory, which yielded confirmation of the reliability of the thermometer.

[12] Callendar left Cambridge and spent two years working as a professor of physics at Royal Holloway College from 1891-1893.

[2] Callendar developed a formula for the heat of steam, expressing it as a function of pressure and temperature, publishing it in his first paper on thermodynamics in 1900.

[1] After Callendar’s death the following year, his son Guy Stewart continued to work on steam properties.

[2] In 1902, Callendar and Barnes developed the continuous-flow calorimeter, used for measurement of the heat capacity properties of liquids.

[2] They used their apparatus to research the comparison of electrical and thermal units, calculating a value for one calorie that compares very closely to the currently accepted value.

[1] Their apparatus was used widely for the determination of the heat capacity of liquids and gas, as well as study of combustion gases.