The outlines of physics: an elementary text-book . h boils at 76 If we mark the ice point 0° and the steam point 100°,the scale will be the centigrade scale. If we continue to Fig. 143. NATURE AND EFFJECTS OF HEAT 159 divide the tube, using degrees equal to 3^, the distance between the ice point and steam point, the bottom of the tube will coincide with scale division —273° C. If we adopt this mark as the zero of our scale and divide the entire tube into degrees of the same size as before, the ice point becomes + 273° and the steam point + 373°. The zero thus chosen is called the
The outlines of physics: an elementary text-book . h boils at 76 If we mark the ice point 0° and the steam point 100°,the scale will be the centigrade scale. If we continue to Fig. 143. NATURE AND EFFJECTS OF HEAT 159 divide the tube, using degrees equal to 3^, the distance between the ice point and steam point, the bottom of the tube will coincide with scale division —273° C. If we adopt this mark as the zero of our scale and divide the entire tube into degrees of the same size as before, the ice point becomes + 273° and the steam point + 373°. The zero thus chosen is called the absolute zero, and the scale is called the absolute scale of the air thermometer. w 141. The Air Thermometer.— It is not practicable to givethe air thermometer the sim-ple form suggested in theprevious paragraph. The es-sential features of the actualinstrument are shown in These are a bulb ofglass or porcelain (J.), whichcontains the dry gas (usuallyair or hydrogen), the ten-dency of which to expand isto be utilized. The neck of. Fig. 144. this bulb is very narrow, and it ends in one arm of themanometer ?Wj, m^. The other arm (m^ is long, and isclosed at its upper end. The space (v) above the mer-cury in this arm contains no air, so that the manometermay be considered as a siphon barometer in which thevertical distance h, between a and b, measured in centi-meters, gives the pressure exerted upon the gas in the gas is heated it expands, driving the mercury 160 TBI: OUTLINES OP PHYSICS downwards at a and upwards at b. This movement iscounterbalanced by raising the cistern 0 until the levelat a is restored. The pressure, as indicated by the differ-ence of level A, necessary to maintain the mercury at thefixed point a, when the temperature of the gas is t°, ascompared with that necessary to give the gas the samevolume at the temperature ^q, measures the change oftemperature, ^ — ^o- This is called the method of constant volumes. The advantage of the air
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