Full Text
On the Applicability of Soot and Platinum Black for Blackening in Measurements of Radiant Energy
(W. Gerlach, Über die Verwendung von Ruß und Platinmohr als Schwärzungsmittel des Empfängers bei absoluten Strahlungsmessungen. Ann. d. Phys. 50, p. 245, 1916.)
The deviation of the properties of the absorbing surface of a bolometer, thermoelement, and other instruments from the conditions of an ideal “absolutely black” surface serves as a source of considerable error in precise, and especially absolute, measurements of radiant energy. Generally speaking, any discontinuity in the optical properties at the boundary between an absorbing surface and the surrounding medium entails a partial reflection of energy. Various materials used for blackening—soot, platinum black, lampblack, and others—possess a rather significant reflecting capacity. The work of Coblentz1 and of Coblentz2 has established that the percentage of energy reflected by surfaces blackened with soot and platinum black ranges from 0.5% to 8%, depending on the conditions of deposition and on the wavelength of the incident radiation. Knowledge of this circumstance is essential in precise measurements of the distribution of energy in the spectrum. A second source of error is the temperature difference between the blackening particles and the metal owing to their different thermal conductivities. Kurlbaum3
showed that, in the case of blackening the metal with a layer of soot of \(1.3\ \mathrm{mg}/\mathrm{cm}^2\) and raising the temperature by \(4^\circ\), the temperature difference reaches \(1.7\%\); on the contrary, for platinum black the temperature difference is negligible.
W. Gerlach points to an entirely new fact—the dependence of the thermal conductivity of soot on the pressure of the surrounding medium. The measurements were made with a thermoelement and a bolometer by the compensation method of Angstroem and Kurlbaum1. The instruments were placed in a vessel in which the pressure could be varied from \(0.001\ \mathrm{mm}\) to one atmosphere. By heating the instrument in turn with incident radiant energy and with an equivalent current, Gerlach found that the deflections of the galvanometer differ considerably for the two kinds of heating, and that the ratio of the two deflections varies with pressure. Apparently, the “thermal insulation” of the layer of soot in the pressure interval from \(0.2\) to \(0.6\ \mathrm{mm}\) attains a maximum and, consequently, when the metal strip is heated by current its temperature rises comparatively, while when it is heated by radiation it falls. In the drawing, on a conventional scale, are shown the results of Gerlach’s experiments with a bolometer.
Fig. 1.
Along the abscissa axis are plotted the pressures; along the ordinate axis, the relative values of the measured radiation energy. Curve 1 corresponds to the case of blackening the bolometer with platinum black, curve 2 to soot in an atmosphere of air, and curve 3 to soot in an atmosphere of hydrogen. It must be noted that the ordinates of the curves are proportional to the ratio of heating by radiation and by current, and therefore the normal change in the sensitivity of the bolometer with pressure is automatically taken into account. From the drawing it is seen that: 1) the absolute measurements by a bolometer blackened with platinum black are entirely independent of pressure; 2) the sensitivity of the bolometer has a sharp minimum in the pressure interval from \(0.2\) to \(0.6\ \mathrm{mm}\); 3) at very low pressures and high pressures \(>200\ \mathrm{mm}\), the sensitivity does not depend on pressure.
4) the decrease in sensitivity is especially sharp in an atmosphere of hydrogen; 5) the error of absolute measurements may reach 60%. A series of Gerlach’s experiments showed that the observed minimum is not connected with selective absorption by vapors which might be released from soot at a definite pressure—the phenomenon is apparently connected with a change in the state of aggregation of soot with pressure (cf. the generally known microphonic properties of charcoal). It should be noted that when the direction of the pressure change is reversed, the curves retain their former form.
Gerlach’s experiments show quite clearly that the (very widespread) use of soot for blackening is entirely inadmissible in precise measurements of radiant energy.
S. Vavilov.