On the use of the N.P.L. photoelectric pyrometer to establish the temperature scale above the gold point (1063 °C)

The design and performance of the N. P. L. photoelectric pyrometer is described together with its use in the realization of the International Practical Temperature Scale over the range 1063 to 2700°C. The photoelectric pyrometer operated on the substitution principle having as an external reference source a black-body lamp. The effective wavelength of the system was defined by an interference filter having a maximum transmission at about 660 nm and a bandwidth of 12.5 nm. A black-body cavity enclosed in a stainless steel-clad copper cylinder was set at the temperature of the gold point using a calibrated Pt/Pt-10% Rh thermocouple. The sectored disks used for measuring the energy ratios were calibrated by the timeratio method which is here described in detail. The new scale thus determined is estimated to reproduce the I. P. T. S. to ±0.1°C at the gold point, ±0.25°C at 1550°C and to + 1.0°C at 2700 °C.