Investigations in absolute radiometry

Abstract

Of modern British work seeking to establish a scale of radiation intensity the most important is that of the late Professor H. L. Callendar (1910), who developed a very accurate instrument, the Callendar Radio-balance, for measuring radiation. Subsequently Callendar used the radio-balance to calibrate various types of radiometer produced by British instrument makers. After his death, arrangements were made for this work to be continued at the National Physical Laboratory. The instrument used to establish a scale of radiation intensity at this Laboratory was a radio-balance made to an improved design of Callendar’s shortly before his death. It was made in the Physics workshops of the Imperial College of Science and calibrated at the National Physical Laboratory as described in the second of these papers. Other scales, notably those known as the Smithsonian Scale of 1913 and the Ångström Scale, have also been employed in this and other countries, particularly for the calibration of meteorological instruments; and it has been recognized for some time that these scales are not in agreement. The discrepancies indicated the importance of obtaining independent evidence of the accuracy of the N. P. L. scale rather than continuing to rely on the properties of a single instrument. Accordingly, the first steps taken in a long investigation, which ultimately led to complete justification of the faith reposed in the radio-balance, concerned the development of absolute radio-meters of an entirely different type. Descriptions of these new instruments, and of the method of using them, form the subject of this first paper.