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Page:Popular Science Monthly Volume 18.djvu/383

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change which takes place in the first years after manufacture has ceased, the freezing-point which is then determined is called the permanent freezing-point, and is the zero of the Centigrade scale, or 32° of the Fahrenheit scale. If we heat the thermometer to the boiling-point of water, and then immediately cool it and immerse it in melting ice, we shall obtain another point on the thermometer scale which we may call the temporary freezing-point, because it will gradually approach the permanent freezing-point, and after a few months, if the instrument is not again heated, it will finally coincide with it. The difference between the permanent and temporary freezing-points is usually about three fourths of a degree Fahrenheit, and, so far as now known, remains constant for the same thermometer.

5. The boiling-point of water at the level of the sea, and with a barometric pressure of 760 mm. 29∙922 inches in the latitude of 45°, is the second point in the thermometer scale to be fixed. To do this the thermometer is exposed to the steam of pure water, and, from the observed height of the barometer, the known elevation and latitude of the place of observation, the true boiling-point is computed from the observed one, and the 100° C. or 212° F. is thus fixed.

6. Having thus located the freezing and boiling points of a standard thermometer, the intermediate points are to be fixed by dividing the scale so that at every part the length of 1° shall measure an equal

Fig. 1.
PSM V18 D383 Thermometer accuracy comparator.jpg

volume of mercury. At least, this has been the usual procedure, and for ordinary standards perhaps it is the most convenient. For standards to be used in the highest class of work it would be better to graduate the distance between 0° and 100° C. into one hundred equal parts, and then allow the observer to accurately determine the value of the corrections at each degree. Indeed, it is preferable for many