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. 5-11.
 
Copyright © 1998 Richard A. Paselk
 


 Construction of a Thermometer.--The first thing to be attended to in this connection, is the selection of the tube to be used for the stem of the instrument. As to the bore of the tube, it can easily be seen that the smaller this is, the greater will be the sensitiveness of the thermometer, assuming the bulb to have the same size in all cases; or, on the other hand, for a given sensitiveness, the size of the bulb can be diminished, according as a tube of a finer bore is employed. The exact relation between the bore and the size of the bulb may be determined as follows :-

 
[calculations for examples with spherical and cylindrical bulbs]
. . .
 
Thermometer Tubes should be of Uniform Bore.--In selecting a tube for the construction of a thermometer, it is most important to determine whether the bore is uniform throughout the length to be used. This point may be settled by carefully sucking a short thread of mercury into the tube, and measuring its length when occupying various positions in it. A tube in which these lengths vary by more than a very small amount should be discarded.
The bulb of the thermometer is sometimes blown directly from the glass composing the tube, but more often is made independently and fused on to the stem. Before doing so, the inside of the tube is carefully cleaned, as any traces of dust or other foreign matter will subsequently cause great trouble and annoyance.
Filling the Thermometer.--Before the bulb has been sealed on to one end of the stem, a thistle funnel A is blown on the other end of the latter. The tube is also drawn out at the point B where the thermometer is to be sealed off. In doing this, care must be taken to pull the tube out as little as possible, but to allow the glass to collapse so as to leave only a very fine aperture, the walls remaining thick. It is further worth while to blow a small expansion, C, at a point just above the position selected for the graduation marking the highest temperature which the thermometer is required to measure. By this means accidental breakage of the thermometer through a small overheating is guarded against. The funnel A having been filled with pure dry mercury, the bulb D is slightly heated so as to drive out some of the Imprisoned air. On allowing D to cool, mercury will be drawn in. Amateurs often heat the bulb too much to start with, resulting in a breakage due to the cold mercury suddenly cooling the hot glass. When once a small amount of mercury has been drawn into the bulb there is less fear of this mishap, since the bulb is then not likely to be heated to a greater temperature than that of boiling mercury. Subsequent heatings and coolings will suffice to entirely fill the bulb and stem with mercury. Finally the whole of the contained mercury must be boiled. This cannot he done without considerable risk when nothing further than a naked flame is used. Greater safety is attained by placing the thermometer, together with its attached thistle funnel filled with mercury, in an enclosure which can be heated to a sufficiently high temperature, and subsequently allowed to cool gradually. The mercury is boiled in order to drive off the air which otherwise always clings to the walls of the tube.
In order to seal the thermometer off, the mercury in the bulb and stem is raised to a temperature sufficiently above that which is to correspond to the highest graduation; this temperature will be that at which the thermometer will burst after being sealed off. A small pointed flame is then directed on to the constriction B, Fig. 4, and the mercury having been evaporated from the neighborhood of the point of the flame, the temperature is increased till the tube fuses and the walls fall together, when the upper part may be pulled off.
Determination of the Fixed Points.--The determination of the fixed points of a thermometer should be postponed for at least a week after the thermometer has been filled and sealed. The most convenient fixed points for a thermometric scale are those corresponding to the melting of pure ice in distilled water, and the boiling point of water at standard atmospheric pressure.
In order to determine the freezing point, the lower part of the thermometer is surrounded with ice shavings, or freshly-fallen snow ; a better result is thus obtained than when ice in the a Thermometer. form of small blocks is used. For a very accurate determination, distilled water contained in a test tube is frozen round a piece of copper rod, and the latter having been removed, the thermometer bulb is placed in the aperture so formed, the space between the bulb and the ice being filled with distilled water. The whole is then placed in an inverted funnel filled with ice shavings, and left for a space of half an hour or so. The position of the extremity of the mercury column may then be marked, and if it is found that no alteration in its position takes place in about ten minutes, this point may be taken as the freezing point, or zero of the thermometer.
In order to facilitate marking the position of the freezing point, a thin layer of varnish or paraffin wax may be laid on the stem ; a scratch in this may be subsequently etched into the glass by means of hydrofluoric acid gas.
 
EXPT. 4 .... [An experimental determination of the zero point of a thermometer using procedure above.]
 
The correct determination of the boiling point of a thermometer is a matter of greater difficulty. It is best to mark the position of the extremity of the mercury column when the bulb and stem are surrounded by steam, at the same time noting the height of the barometer. A correction can then be calculated, giving the amount by which the graduation so obtained is removed from the boiling point under standard atmospheric pressure.
The apparatus used for the determination of the boiling point is shown in Fig. 6. It is best to provide the cork, through which the thermometer is thrust, with a rather large hole, the thermometer being prevented from slipping through by a ring cut from a piece of india rubber tubing fitting tightly on it. The whole of the stem as far up as the extremity of the mercury column should be surrounded by steam. When the extremity of the mercury column has attained a position which does not alter during five or ten minutes, it can be marked by a scratch, and the barometer immediately read. The thermometer tube may now be graduated. A coat of paraffin wax having been laid evenly over the stem, the distance between the fixed points is divided into 100 equal parts (200 parts if half degree graduations are employed). The positions of the graduations are marked by scratchs in the wax. They may be etched into the glass by hydrofluoric acid, the vapour being used in preference to the liquid.


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Last modified 22 July 2000