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The Emission Constants of Metals. 467 of the later ones have since been controverted. Nevertheless, it can be stated without doubt that the variation of metallic emissivity with temperature is considerably less in the visible than it is in the region of validity of the Hagen- Rubens relation.
The Emission Constants of Metals. 469 observed black-body temperatures being corrected to surface temperatures by using Holborn and Henning’s* results for the visible emissivities—results which have been adversely criticized by Worthing.t At lower temperatures, deflections of only a few millimetres were obtained, and it is hard to see how the accuracy of a few per cent, claimed by the authors for all their results can be justified. McCauley admits that, as his black-body curves were computed from a knowledge of the emissivities of the metals at a part of the spectrum where bolometric measurements are uncertain to as much as 10%, his values of the reflecting power are apt to be in error. His results have also been criticized by Coblentz,J who likewise objects to the indirect method of obtaining the data.
The Emission Constants of Metals. 471 7*5 mm. X 1 mm. base, so that, when the strip was hammered into the saw-cut in the emitter and riveted into position, a cavity with the shape of a Mendenhall wedge, 1*5 cm. long, 1 mm. broad at the orifice and 7-5 mm. deep, was formed. The angle of the wedge was therefore 7° 36'. A rough file was used on the sides of the V-cavity in order to reduce the reflection coefficient of the walls by the formation of scratches parallel to the length of the cavity, each of which would blacken the radiation by internal reflection.
The Emission Constants of Metals. 473 material. During the course of the work on copper, a dull black paint, drying with a matt surface, was used. This paint, supplied by British Drug Houses, Ltd., is chiefly composed of an extract of lampblack in turpentine. After the application of the paint, the turpentine was driven off by gentle heating in a current of air. The carbon content of the paint rendered it unsuitable for use while nickel was under investigation. It was replaced by platinum black, which was deposited on thin copper foil fixed to the walls of the container.
The Emission Constants of Metals. 475 The linearity of the galvanometer scale was tested for each new setting of the control magnets by means of a potentiometer system. With few exceptions, it was found that, so long as the deflection did not exceed about 15 cm., it was accurately proportional to the current passing through the galvanometer.