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By thermometer testing we mean determining the indications of the thermometer which correspond to the freezing point of water and to its boiling point under a pressure of 760 mm.
The first observation is made by placing the thermometer so that its bulb and stem up to the zero are surrounded with pounded ice. The ice must be very finely pounded and well washed to make quite sure that there is no trace of salt mixed with it. This precaution is very important, as it is not unusual to find a certain amount of salt with the ice, and a very small amount will considerably reduce the temperature.
The boiling point at the atmospheric pressure for the time being may be determined by means of the hypsometer, an instrument described in any book on physics.(1)
The thermometer to be tested must be passed through the cork at the top of the hypsometer, and there fixed so that the 100° graduation is just above the cork. One aoerture at the bottom of the cover of the hypsometer is to allow the steam issuing from the boiling water to escape; to the other aperture is attached by an india-rubber tube a pressure gauge, which consists of a U-shaped glass tube containing some coloured liquid. The object of this is to make sure that the pressure of the steam within the hypsometer is not greater than the atmospheric pressure.
The water in the hypsometer must be made to boil and the thermometer kept in the steam until its indication becomes stationary. The temperature is then read.
In each of these operations, in order to make certain of avoiding an error of parallax in reading (i.e. an error due to the fact that since the object to be read and the scale on which to read it are in different planes, the reading will be somewhat different according as the eye looks perpendicularly on the stem or not), the thermometer must be read by a telescope placed so that it is at the same height as the graduation to be read. If, then, the thermometer be vertical, the line of sight being horizontal will be perpendicular to it (It must be remembered in estimating a fraction of a division of the thermometer that in the telescope the image 'of the scale is inverted.)
We thus determine the boiling point at the atmospheric pressure for the time being. We have still to correct for the difference between that pressure and the standard pressure of 760 mm. To do this the height of the barometer must be read and expressed in millimetres. We obtain from a table showing the boiling point for different pressures, the fact that the difference in the temperature of the boiling point of 1° corresponds to a difference of pressure of 26.8 mm. We can, therefore, calculate the effect of the difference of pressure in our case.
Suppose the observed boiling point reading is 99.5, and the height of the barometer 752 mm. We may assume that, for small differences of pressure from the standard pressure, the difference in the boiling point is proportional to the difference of pressure; hence
And therefore the corrected boiling point would read 99.8° on the thermometer.
The correction is to be added to the apparent boiling-point reading if the atmospheric pressure is below the standard, and vice versa.
(1) Determine the freezing and boiling points of the given thermometer.
Enter results thus:
Thermometer, Hicks, No. 14459. Freezing point -0.1° Boiling point 99.8°The following additional experiments may be performed with the hypsometer.
(2) Put some salt into the hypsometer and observe the boiling point again.
(3) Tie some cotton wick round the bulb of the thermometer, and let the end drop into the solution. Vide Garnett, §13. (The cotton wick should be freed from grease by being boiled in a very dilute solution of caustic potash and well washed.)
(4) Remove the water, clean the thermometer, and repeat the observation with a given liquid
Boiling point of alcohol is 79°, b.p. of ether 37°, b.p. of turpentine 130°.
(5) Clean the thermometer and hypsometer, and remove the apparatus to a room in the basement, and observe the temperature of the boiling point of water.
Take the apparatus up to the top of the building and repeat, and from the two observations determine the height of the building thus:
The difference of temperature of the two boiling points depends only on the difference of pressure. Also an increase of pressure of i mm. of mercury produces an alteration of the temperature of the boiling point of 0.0373°C, or an increase of temperature of the boiling point of 1° corresponds to a pressure of 26.8 mm. of mercury.
Now the specific gravity of mercury referred to water is 13.6, that of dry air at 760 mm. pressure, and 15° C temperature is 0.001225. Thus the pressure due to 1 mm. of mercury is equal to that due to 13.6/0.001225 mm, or 11.102 metres of dry air.
But a rise in temperature of 1° corresponds to an increase in pressure of 26.8 mm. mercury; that is, to an increase of pressure due to 11.102 x 26.8 metres of dry air.
Thus, the boiling point alters by 1° C for an alteration of pressure equal to that due to a column of dry air at 15° C and of 297.5 metres in height.
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