. Journal. peraturecoefticient of electric resistance of platinum is aslarg- as the expansion coeflicient of gas. we wouldejcpect the resistance to double below 800 at ooO~ C. qundrviple at 800^ C. and so we insert the thermocouple deeply enough intothe furnace to prevent the coolin;^ of the junctionby the heat conduction of the metallic wire com-posing the thermocouple, the additional resistanceamoimts sometimes to 4—0 ohms, which nieansan error of •—10~ C. at TOO C. The ordinarypyrometer is usually so graduated that it readstrue temperat\u*s when tlie thermo*ouple has noresist
. Journal. peraturecoefticient of electric resistance of platinum is aslarg- as the expansion coeflicient of gas. we wouldejcpect the resistance to double below 800 at ooO~ C. qundrviple at 800^ C. and so we insert the thermocouple deeply enough intothe furnace to prevent the coolin;^ of the junctionby the heat conduction of the metallic wire com-posing the thermocouple, the additional resistanceamoimts sometimes to 4—0 ohms, which nieansan error of •—10~ C. at TOO C. The ordinarypyrometer is usually so graduated that it readstrue temperat\u*s when tlie thermo*ouple has noresistance. Even when the pyrometer and thethermocouple are standardised and calibratedbeforehand at the fixed points of the themio-metric scale (see below), this error can only bepartially eliminated, for even at the fixed tem-perature, the temperature readings will coincideonly when the temperature distribution along thethermocouple is such that the electric resistanceof the thermocouple remains Fig. 4. Another source of error in the uncompensatedmethod is the temperature change of the junctioncaused l)y the thermo-electric current itself. Thiserror is, however, less serious than the former. Both the sources of error mentioned above canbe avoided by the method—a methodof measurement of thermo-electric power withoutflow of current. A circuit containing two accu-ratelv standardised resistances, t, and r^, andauxifiary cells, A (Fig. 4), has two shunts in r,and Tj. the one containing a normal cadmium cell,X, and a galvanometer, Ci. the other containinga thermocouple and a galvanometer, (!,. Theresistances, ri and r,. are so regulated that thereis no flow of electric current in either we have 6 = 10184 xL*. Fig. 4 shows the arrangement used in the caliVjra-tion of the thermocoupfe. The cold junctions areput in a few drops of mercurj contained in testtubes, which latter are immersed in ice. For of the arms of the thermocouple a
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectchemist, bookyear1882
