Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

from : Edwin Edser. Heat for Advanced Students. Macmillan and Co., Limited. London (1911) pp. 14-16.
Copyright © 1998 Richard A. Paselk

 Sensitive Mercury Thermometers.--In order that it be possible to read a temperature with great accuracy, it is obvious that the extremity of the mercury column should move through a considerable distance along the stem for a small alteration of temperature. Consequently, if such a thermometer is to be used for measuring temperatures from 0° C. to 100° C., either a very long stem must be provided, or some modification of the usual form must be employed.

The most usual procedure is to furnish the upper extremity of the thermometer tube (which is bent over as shown in Fig. 8), with an enlargement into which part of the mercury can be driven by heating. A sufficient amount of mercury must be left in the bulb and stem to give readings between the required temperatures. Fig. 8 represents a thermometer of this description. The following points in its construction may be noted. In order to avoid the errors due to the irregular motion of the mercury in a very fine tube, a tube of comparatively large bore is employed. It has already been pointed out that the sensitiveness of a thermometer depends on the ratio, Volume of bulb: Sectional area of bore of tube, and as the bore is made comparatively large, a very large bulb, is required. The stem is provided with an enlargement into which part of the mercury can be driven, for reasons explained above. The thermometer tube is made with comparatively thin walls, and to protect it from injury it is contained within a wider tube, which is fused at its lower extremity on to the bulb. The graduations are marked on a separate enamelled scale placed behind the thermometer tube, and inclosed in the outer guard tube. In order that this thermometer should be capable of furnishing readings for very quick changes of temperature, the walls of the bulb must be made very thin. This of course will expose it to considerable errors due to variations of pressure. (See Chapter II).
The fixed points of such a thermometer obviously cannot be obtained in the manner previously described. Its scale must be calibrated by comparison with a standard thermometer. Such a thermometer as that considered is, however, more often used to measure small changes of temperature than to determine actual temperatures.

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