Book - School of Science and Technology

TemperatureMethods of measurement 13The readings taken from an ordinary mercury-in-glass thermometer provide, in general,temperature values relating to the gas, liquid or solid in which the instrument is immersed.When applied to the air volume within a building, however, the situation is much morecomplex. Here, the scale reading will portray a situation of equilibrium, taking account ofnot only air temperature but also of a variety of heat exchanges between surroundingsurfaces. These latter may be partly by conduction, partly by convection and partly byradiation.If such a thermometer were to be immersed in a hot liquid, conduction and convectionwould account for practically the whole effect, but if the same instrument were used toprovide the temperature of room air, then the effect of radiation might predominate.Thus, in summer, the sun temperature might be 37 C, while the shade temperaturereached only 27 C:the air temperature in each case might well be the same but in theformer case the thermometer would be exposed to the sun, while in the latter case it wouldnot.In practice, the nature of the source of radiant heat affects questions of measurement.The so-called diathermic property of glass, in this case the bulb of the thermometer,permits the inward passage of high temperature radiation but is impervious to an outwardpassage at lower temperature. A radiation shield, which may be no more than a piece ofaluminium cooking foil shading the thermometer bulb, will negate this influence andpermit air temperature alone to be read.Measurement of radiant effects thus depends upon a whole variety of circumstances.A solar thermometer, for measurement of high temperature radiation, consists of aglass sphere and tube within which a vacuum has been drawn, containing a simplemercury-in-glass instrument having a blackened bulb, as Figure 1.2(a). Similarly, a globeSpacerTube graduatedin degreesOrdinarythermometerMercurycapillaryLoose-fittingstopperBlackenedcopperglobeOuter glasstubeSpacerBlackenedbulbVacuum space(a)(b)Figure 1.2 Solar (a) and globe (b) thermometers