. The Philosophical magazine; a journal of theoretical, experimental and applied physics . )ted. Their capacity is somewhat over 300 cub. centim.,and the mean thickness of the porcelain walls about 027centim. The stem, which is, say, 40 centim. long, has avolume of about 0*012 cub. centim. per centimetre. To use such a bulb for calibrating a thermocouple, it wouldbe necessary to have a space at an almost unattainably constanthigh temperature : for the couple is practically a point, andits thermal sensitiveness is instantaneous, whereas the largebulb necessarily lags behind changes of temperatu
. The Philosophical magazine; a journal of theoretical, experimental and applied physics . )ted. Their capacity is somewhat over 300 cub. centim.,and the mean thickness of the porcelain walls about 027centim. The stem, which is, say, 40 centim. long, has avolume of about 0*012 cub. centim. per centimetre. To use such a bulb for calibrating a thermocouple, it wouldbe necessary to have a space at an almost unattainably constanthigh temperature : for the couple is practically a point, andits thermal sensitiveness is instantaneous, whereas the largebulb necessarily lags behind changes of temperature. Withthe object of eliminating this source of serious error, I hadthe bulbs made in the shape of fig. 2, the bottom of whichis reentrant, projecting inward to form an axial tube, m n, Fig. 2.—Xou-iiiglazed reentrant porcelain air-thermometer 1 closed at the centre of figure, m. Into this tube the end ofthe properly insulated thermocouple is introduced, with itsjunction at m. The insulator, § 10, incidentally subservesthe purpose of a plug for the open end of the tube ni7i, B2 4 Mr. C. Barns o)i the Fusion whereby radiation here is made imperceptible. In this re-entrant pattern symmetry of form has been sacrificed in orderto secure gi-eater identity in the exposure of the air-ther-mometer and the thermocouple. Deville and Troost condemnthe non-spherical bulb, but I have not in my experiments withthe reentrant form found any evidence to agree with all events the expansion error is much less serious thanthe calibration error in question (§ 14). The amount of gas or moisture forcibly retained by thelucres of the non-inglazed porcelain bulb may be greater thanthe manometer pressure indicates. There is apparent difficulty,moreoA^er, in defining the interval volume, a quantity whichin constant-pressure air-therniometry e
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