however, the measurements are based on the arbitrary selection of two standard temperatures, and the subdivision of the interval between them into a convenient number of divisions or degrees.

FIG. 1.-Mercury
Thermometer.

The methods used in measuring temperature, and the precautions to be adopted in constructing a thermometer, are well illustrated in the construction and use of a mercury thermometer, which will now be described.

Mercury Thermometer.-When mercury is heated, its volume increases; consequently, if a method of measuring this increase of volume can be arranged, the temperatures corresponding to any given increase can be defined.

Let us suppose that we have a bulb blown on the end of a glass tube, and that the bulb and part of the tube are filled with mercury. Let us further assume, for the moment, that the volume of the bulb and tube remains unaltered as the temperature is varied. Then, if the mercury be cooled to the temperature of melting ice, by immersing the bulb and the part of the stem filled with mercury in ice-shavings or snow, position occupied by the mercury meniscus may be marked on the stem, thus giving one fixed temperature.

the

It is known that a pure liquid boils at a constant temperature as long as the pressure of its vapour is maintained constant. Consequently, the temperature of the vapour above pure water, boiling under some arbitrary pressure, will be constant, and we may define this as the second fixed point on the thermometric scale. The bulb and that part of the tube containing mercury are placed in steam, and the position of the extremity of the mercury column is marked on the stem.

If the bore of the tube is uniform along its whole length, then we may divide the space between these two marks into any convenient number of equal spaces--one hundred such spaces are marked on an ordinary

Centigrade thermometer. When the mercury meniscus is opposite any one of these, we may state that the temperature of the mercury has a certain value; and if the conditions of the experiment are so arranged that the thermometer thus constructed has had an opportunity of acquiring the temperature of the surrounding medium, we may thus determine the temperature of the latter.

We have tacitly assumed, up to the present, that one end of the thermometer tube is open to the atmosphere. The disadvantages of this arrangement are :

1. Liquids or foreign substances might enter the tube, and thus cause trouble.

2. Mercury might leave the tube; this would certainly happen, since mercury slowly evaporates, even at ordinary temperatures.

Consequently, an advantage will be gained by closing the end of the tube. As a general rule, the space above the mercury is freed from air; if this were not so, the pressure produced by the compression of the enclosed air by the expanding mercury would break the walls of the bulb, unless these were very thick.

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:

Let = volume of bulb. (If the bulb is spherical, v =

TR3, where

Let a internal sectional area of tube. (If the section of the bore is circular, a = r2, where r = internal radius of the tube.)

Let x =

distance that the end of the thread of mercury in the stem moves through for a rise of temperature of 1° C.

Let a = the increase in volume experienced by unit volume of mercury when heated through 1o C.

The increase in the volume of the mercury contained by the bulb,

when the temperature is raised by 1° C = va. This mercury flows into the stem, and fills an extra length x of the latter.

EXAMPLE. It is required to construct a mercury thermometer from tubing of circular bore, the internal diameter being equal to 2 mm. What must be the diameter of the bulb blown (taking a = '000181 per 1° C.) in order that the distance between consecutive degree divisions should be equal to 2 mm.?

This example shows that in order to obtain reasonable sensitiveness, combined with a bulb of moderate size, a tube of very small bore must be used.

Thermometers with Cylindrical Bulbs.--Ordinary chemical thermometers are made with cylindrical bulbs, so as to facilitate their introduction into flasks, &c., through small apertures. A not uncommon length for such a bulb would be 20 mm., the diameter being about 3 mm. Let us now determine what must be the diameter of the tube employed, in order that successive degree divisions should be 2 mm. apart.

Hence, if

x 2. mm.

V = π × (15) × 20 c. mm.

internal radius of the tube employed, which is supposed

to have a circular bore,

π2 × 2 = π × (.15) × 20 × 000181.

1-2 = *000040689 mm.

700637 mm.

... Internal diameter of tube = 01274.

110

This bore is so small that great difficulty would be experienced in noting the position of the end of the mercury thread. In order to overcome this difficulty, tubing with an elliptical or flattened bore (Fig. 3) is generally used. Thus, when the mercury is viewed at right angles to the longer axis of the ellipse, the position of the end of the column is plainly visible. The glass itself serves to furnish a magnified image of the mercury column. In addition, a layer of white enamel is frequently embedded at the back of the tube; thus giving a good background for viewing the mercury.

100

50

FIG. 2.-Chemical
Thermometer.

FIG. 3.-Section of Stem of Chemical Thermometer.

In chemical thermometers of the class under discussion, the graduations are generally engraved on the front or clear glass surface of the thermometer tube. In taking readings, it is necessary to carefully avoid parallax. This can be done with great accuracy by placing the eye in such a position that the engraved divisions in the neighbourhood of the top of the mercury column are seen just to overlap their reflections in the mercury.

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.

A

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.

B

C

D

Thermometer in Course of Construction.

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. FIG. 4.-Chemical Finally the whole of the contained mercury must be boiled. This cannot be 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