Page:International Library of Technology, Volume 93.djvu/31

 temperature. This expansion or contraction causes the top of the mercury column to rise or fall; and, since equal changes of temperature make the mercury column rise or fall equal distances, the graduations on the scale are made equal throughout.

In Fig. 1 is shown a combination thermometer, that is, one that has two scales. The scale on the left, marked F, forms, in combination with the glass tube, a Fahrenheit thermometer (so named after its inventor), which is the one commonly used; the scale on the right, marked C, forms, in combination with the glass tube, a centigrade thermometer. The centigrade thermometer is used by scientists throughout the world, because the graduations are better adapted for performing calculations.

7. Graduating a Thermometer. — The graduations on the scale of a thermometer are obtained as follows: The thermometer is first placed in melting ice, and the point to which the mercury column falls is marked and called the freezing point; the thermometer is then placed in steam that is escaping from an open vessel, and the point to which the mercury rises in the tube is marked and called the boiling point. These are two fixed points; that is, the mercury column will always register these same points when the thermometer is placed in broken ice or in steam, under the conditions explained above.

8. The freezing point marks the temperature at which, under atmospheric pressure, water freezes and forms ice, or at which ice melts and forms water, since water freezes and ice melts at exactly the same temperature, as heat is abstracted or added.

The boiling point marks the temperature at which water boils and forms steam under atmospheric pressure.

Having fixed these two points, the distance between them is divided into equal parts, the number of divisions depending