. Science of railways . G e<Xcje LOCOMOTIVE APPLIANCES, 191 base and running high in the air (see Fig. 1). Thepressure from the test pump is apphed to the surface of. Fig. Gauge Tester. the mercury and causes the latter to rise in the tube toa height proportionate to the pressure apphed. At 192 LOCOMOTIVE APPLIANCES. each point of graduation on the tube, an insulatedplatinum wire point is inserted and connected with anelectrical register. By means of battery connectionsto the mercury and to each platinum point, when thetwo come in contact at each graduation, the electric
. Science of railways . G e<Xcje LOCOMOTIVE APPLIANCES, 191 base and running high in the air (see Fig. 1). Thepressure from the test pump is apphed to the surface of. Fig. Gauge Tester. the mercury and causes the latter to rise in the tube toa height proportionate to the pressure apphed. At 192 LOCOMOTIVE APPLIANCES. each point of graduation on the tube, an insulatedplatinum wire point is inserted and connected with anelectrical register. By means of battery connectionsto the mercury and to each platinum point, when thetwo come in contact at each graduation, the electricalregister is formed. Although the sealed tube was allowed on higherpressures, yet as late as 1843 the French governmentrequired the open tube to be used for engines undersixty pounds and steamboats under thirty poundspressure per square inch. On account of its defects and disadvantages forordinary pressure measurement, the tube of mercurygradually gave way to gauges of mechanical con-struction, more suitable for practical use. Thedesigns and modifications of such mechanical gaugesare now quite numerous, but for years there were twoprincipal types, viz.: the Bour
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