Misurazione di temperatura con resistenza in platino a temperature elevate . tes, durante il quale intervallo, fora metallo puro, la temperatura dovrebbe di solito rimangono costanti toa di pochi centesimi di grado. A temperature più elevate itwas ritenuto desiderabile per impedire il surriscaldamento del therrnometerhead al fine di ridurre al minimo il thermal CEM (). Thiswas effettuato facendo passare una corrente di aria attraverso la doppia-walledhead del termometro. Quando questo è stato fatto la testa couldbe afferrata dalla mano per agitazione e senza fastidi. Allo scopo di evitare la rottura della porcellana il tubo di contenimento è stato preheatedin un forno a gas a BEF

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Platinum resistance thermometry at high temperatures . tes, during which interval, fora pure metal, the temperature would usually remain constant toa few hundredths of a degree. At the higher temperatures itwas deemed desirable to prevent overheating of the therrnometerhead in order to reduce to a minimum the thermal emfs. Thiswas effected by passing an air current through the double-walledhead of the thermometer. When this was done the head couldbe grasped by the hand for stirring without discomfort. In orderto avoid breaking the porcelain containing tube, it was preheatedin a gas furnace before introducing it into the metal. Crucibles.—Two kinds of crucible were used, of Dixon graphiteand of Acheson graphite, the latter being turned from rods. Thecrucibles were about 13.5 cm inside depth, 5 cm inside diameterat the top, and 3.5 cm at the bottom, holding therefore about i6o Bulletin of the Bureau of Standards. PLATE III {Vol. 6, No 2. THERMOMETER LEADS TO HEATING COILS PORCELAIN TUBES- GRAPHITE CRUCIBLWITH COVER PORCELAIN TUB FIRE CLAY. ALUMINIUM MAGNESIA ASBESTOS fc^m^Mm^m^ -—mmm^msW^mv^^^ Mil WEWL 10 cm CRUCIBLE RESISTANCE FURNACE ¥urg^sT] Platinum Thermometry at High Temperatures. 161 180 cc of metal, or about 1.5 kilograms. A chemical analysis ofthe Acheson graphite, cut from several crucibles, was made byDr. W. F. Hillebrand of this bureau. The total ash was found tobe 0.3 per cent, one-half of which was Fe303 (equivalent to 0.11per cent Fe). No differences in the observed values of any of thefreezing points could be traced to the effects of impurities intro-duced from the crucibles. Surface Oxidation and sublimation of the metals were prac-tically eliminated, even for antimony, by putting a layer ofpowdered graphite and a graphite crucible cover over the surface. Depth of Immersion.—Measurements were usually taken withthe thermometer immersed to within 1 cm of the bottom of thecrucible. The effect of varying depths of immersion was care-fully studied b

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• bookcentury1900
• bookdecade1900