Callendar’s platinum thermometer

The idea that resistance varies with temperature was first aired by Humphry Davy in 1821. Although several scientists followed up this observation, nothing came of it until Charles William Siemens used copper wire to measure the temperature deep inside rolls of cables. Although he had established the principle of resistance thermometry, a committee of scientists rejected the method as too unreliable for serious use. This would change in 1885, when Hugh Callendar joined J J Thompson’s research group in Cambridge intending to develop the resistance thermometer as a new thermometric standard. By 1897 felt confident enough to propose the platinum thermometer, now free of the gas bulb, as a new temperature standard. By 1903 it was in use at the National Physical Lab, and the standard platinum resistance thermometer still defines the international temperature scale today, even after the redefinition of the kelvin.