Fig. 43.—Energy Radiated by a Black Body
at Different Temperatures.
When the relation between temperature and quantity of energy radiated is known, any instrument which will indicate the amount of the radiations it receives may be used to measure temperatures. The ray, for example, may be focused on a thermal junction, which will be heated in proportion to the amount of energy incident upon it, and when connected to a millivoltmeter will cause deflections proportional to the energy it receives. A thin strip of metal might be used in place of a junction, and by measuring its resistance the heating effect of the radiations, and hence the amount thereof, may be deduced. A third method would be to focus the rays on to a compound strip of two metals, which by altering in shape could be made to furnish a clue to the quantity of energy received by it. In theory, it is only necessary to allow the radiations to fall on the working part of any instrument for measuring low temperatures, when the rise in temperature produced may be taken as proportional to the energy received, and the thermal condition of the radiating body deduced from the fourth-power law. In practice, however, it is desirable that the receiving thermometer should be small in size; of low thermal capacity, so as to respond rapidly; and capable of giving a sensitive indication—hence an ordinary mercury thermometer would be unsuitable for this purpose. A thermopile, placed at a fixed distance, would fail owing to the cold junctions gradually warming up by conduction through the pile. The part receiving the radiations should be coated with lamp-black, so that practically all the waves impinging upon it, whether luminous or non-luminous, may be absorbed, and the energy they represent utilised in producing a rise in temperature.
Practical Forms of Radiation Pyrometers: Féry’s Instruments.— In the year 1902 Féry introduced a pyrometer in which the rays were focused by the aid of a lens upon a small, blackened thermal junction, in the same way that the rays of the sun may be focused by a burning-lens. The junction was connected to a special form of d’Arsonval galvanometer, which recorded the E.M.F. developed. By taking the readings of the galvanometer as proportional to the temperature of the junction—that is, to the radiant energy impinging upon it—the temperature of the source could be calculated from the fourth-power law. The drawback to the use of this instrument was the fact that a proportion of the rays was absorbed by the glass, this proportion, moreover, varying at different temperatures, so that the fourth-power law could not be applied with accuracy. By using a fluorspar lens in place of glass, this error was overcome, but the cost of a good lens of this material being high, its use in ordinary workshop practice was rendered prohibitive on account of the price. A number of these pyrometers, furnished with glass lenses, and calibrated by comparison with a standard possessing a fluorspar lens, were placed on the market, but were superseded in 1904, when Féry hit upon the plan of focusing the rays by means of a concave mirror, thus overcoming the error due to absorption by the glass lens. This plan, which serves admirably, has since been adopted in most radiation pyrometers.
Féry’s Mirror Pyrometer.—This instrument is shown in longitudinal and also in cross section in [fig. 44]. A concave mirror, M, which has a gilt reflecting surface, is placed at one end of a metal tube, and is fastened to a rack which engages in a pinion moved by the milled-head, P, so that on turning P, a longitudinal movement is imparted to the mirror. A small, blackened thermal junction, shown at the centre of the cross section, and consisting of a copper disc to which wires of copper or iron and constantan are fastened, receives the rays after reflection, and may be brought into focus by suitably moving the mirror.
Fig. 44.—Féry’s Mirror Pyrometer. Section.