. A text book of physics, for the use of students of science and engineering . II MLEODS PRESSURE GAUGE 435 order to increase the efficiency, several grooves are cut in A, and areconnected with one another so that the low-pressure side of oneis the high-pressure side of the next (Fig. 420 b); there is thus anumber of pumps in series. A preliminary pump is used in orderto reduce the initial pressure to a few millimetres of mercury. TheGaede pump deals effectively with vapours as well as gases, sincethere is no compression duringthe removal and therefore no con-densation of the vapours. Anextern
. A text book of physics, for the use of students of science and engineering . II MLEODS PRESSURE GAUGE 435 order to increase the efficiency, several grooves are cut in A, and areconnected with one another so that the low-pressure side of oneis the high-pressure side of the next (Fig. 420 b); there is thus anumber of pumps in series. A preliminary pump is used in orderto reduce the initial pressure to a few millimetres of mercury. TheGaede pump deals effectively with vapours as well as gases, sincethere is no compression duringthe removal and therefore no con-densation of the vapours. Anexternal view of the pump is givenin Fig. 421. Very low pressures can he ob-tained by first exhausting thevessel as thoroughly as possibleby means of a pump. A tubecontaining coconut charcoal, andin communication with the vessel,is immersed in a bath of liquidair. The remainder of the gas inthe apparatus condenses in, andis absorbed by, the charcoal, thusproducing a very low pressure. MLeods pressure gauge.—The pressure gauge illustrated in is suitable for measuring low. Fig. 421.—-Gaede molecular air pump. pressures. A vertical tube ABCD is sealed at its top end ; the portionAB is of small bore and there is a large bulb between B and C. Thelower end is connected by flexible tubing to a mercury cistern Efurnished with a tap. A branch at C leads to another tube FG, ofthe same bore as AB in order to avoid capillary effects, and thence tothe vessel in which the pressure is to be measured. With the mercury level adjusted so that the branch at C is justclosed, the volume V contained between A and C is known from aprevious calibration, as are also the volumes between A and variouslevels of the mercury in AB. A scale of volumes is attached to AB,zero of the scale being at A. In using the gauge, the level of the mercury is first adjusted so asto be lower than C ; the whole of the space above the mercurysurface is filled with gas at the same pressure as that in the cist
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectphysics
