XXVII.—A Common Thermometric Error in the Determination of Boiling-Points under Reduced Pressure

Extract

When the bulb of a thermometer is enclosed in an evacuated vessel, the dilatation of the bulb introduces a considerable error in the temperature readings. This fact may be well known, but in the literature of boiling-point and vapour-pressure determinations we have observed no reference to it, and no corrections on account of it. Yet, except in the roughest work, this effect cannot be ignored. Thus, a test carried out with eleven thermometers showed that when the pressure round the bulb was lowered from 748 mm. to 20 mm. and thermal equilibrium with the bath had been recovered, the readings were from 0·10° to 0·17° lower. In all but one case, when there was a slight permanent dilatation, the change was constant and was a linear function of the change in pressure. The change bore no relation to the sizes of the bulbs. The thickness of the glass varied considerably, but could not, of course, be measured.