Hugh Longbourne Callendar, English physicist, c 1900.
© Science Museum / Science & Society Picture Library
Callendar (1863-1930) was profesor of physics at McGill University, Montreal; at University College London; and at the Imperial College of Science. Callendar invented an air thermometer with constant presure which could measure up to 450 degrees Celsius. He also made a platinum resistance thermometer which was extremely accurate.