perature rises.

It can be brought back to the mercury surface

by the aid of a magnet.

In Six's self-registering thermometer (Fig. 10) the bulb A is filled with alcohol or a similar liquid, and this extends to B, one surface of a mercury thread BC. Above C there is more alcohol, which partly fills the

bulb D, leaving, however, a space for
expansion.
Two steel indexes, pro-
vided with springs just strong enough
to prevent them from slipping, are
respectively situated above the free
ends of the mercury thread. When
the temperature rises, the index is
pushed before the mercury surface at
C, and is left in position when a fall
of temperature causes the mercury
to withdraw. Thus the maximum
temperature is registered. Similarly,
the minimum index is moved when
the mercury surface at B comes in
contact with it, and is left in position
when a rise of temperature takes
place.

In Negretti and Zambra's maximum thermometer there is a constriction in the tube just above the bulb, so that as the temperature rises the mercury is extruded, whilst a subsequent fall of temperature causes the mercury column to break at the constriction, leaving the mercury in the tube in the position which it occupied when the temperature was a maximum. The maximum temperature is thus read directly from the position of the upper end of the mercury column. The mercury can be shaken back into the bulb when a new reading is to be taken.

Fig. 11 represents a Clinical thermometer constructed in this manner, the temperatures which it can indicate being confined within the limits 95° F. and 113° F.

In Phillips' maximum thermometer (Fig. 12) a short thread of mercury is separated from the rest of the mercury in the stem by a small air bubble. The detached thread acts like the index shown in Fig. 10. The illustration

shows such a thermometer mounted in a strong glass tube for deep sea soundings. The space between the thermometer and the guard tube is partly filled with alcohol, so as to ensure a good thermal communication between the thermometer bulb and the surrounding medium.

High Temperature Ther

mometers.-Under

This

normal pressure mercury boils at 357° C., so that an ordinary mercury thermometer cannot be used above that temperature. inconvenience may to some extent be overcome by filling the space above the mercury with compressed nitrogen, so that the mercury is prevented from boiling by the high pressure to which it is subjected (see Chapter VIII). The bulb must necessarily be made very strong, and even then, if a thermometer of this class is maintained for some time at a high temperature (400° C. or so), the zero will often be found to have changed considerably

when cooling takes place. Metallic Potassium and Sodium when mixed form a peculiar alloy, which presents very much the appearance of mercury, and is liquid at ordinary temperatures. This has been successfully employed in the construction of high temperature thermometers. These may be used at temperatures

considerably above those admissible for ordinary mercury therThe bulb and stem are made of hard Jena glass.

mometers.

Gas Thermometers.-The high expansion of gases renders them especially suitable for thermometric substances, as in their case expansions of the containing vessel produce comparatively small effects on the accuracy of the observed temperature. As, however, the theory of their action depends on certain laws which will be developed later, their consideration is deferred for the present. (See Chapter V.)

SUMMARY

The term "temperature " is used to denote the hotness of a body. Our ideas of temperature are primarily derived from our sensations, but in order to determine temperatures accurately, some physical property which varies continuously with the temperature must be measured.

A thermoscope indicates the attainment of a particular temperature. A thermometer is an instrument designed to measure temperatures. Mercury expands when heated; and this property is utilised in the construction of mercury thermometers. Various forms are given to these instruments, according to the purpose for which they are to be employed.

In all cases two fixed points, corresponding to two arbitrary temperatures, are selected, and the difference between these temperatures is divided into an arbitrary number of degrees.

On the Centigrade scale the temperature of melting ice is taken as o°. The temperature of the steam above water boiling at standard atmospheric pressure is defined as 100°.

On the Fahrenheit scale, temperature of melting ice perature of boiling water = 212°.

=

32°,

tem

On the Réaumur scale, temperature of melting ice = o, temperature of boiling water = 80°.

A sensitive mercury thermometer must either be furnished with a very long stem, or a receptacle must be provided, into which part of the mercury can be driven when occasion requires. The latter procedure is most frequently followed.

A maximum thermometer is used to record the highest temperature attained during a given time.

A minimum thermometer is used to record the lowest temperature attained during a given time.

In high temperature mercury thermometers, ebullition is prevented by subjecting the mercury to a high pressure. In other cases an

alloy of sodium and potassium is substituted for mercury.

QUESTIONS ON CHAPTER I.

(1) Describe some of the principal forms of maximum and minimum thermometers. (S. and A. Adv. (Day), 1898.)

(2) Find the Centigrade temperatures corresponding to 68° F., 176° F., - 20° F., and the Fahrenheit temperatures corresponding to 4° C., 52° C., and -273° C.

(3) What peculiarities of construction may be noticed in a sensitive inercury thermometer?

(4) Explain how you would proceed to determine the error due to the exposed column of a thermometer.

(5) Describe the process of determining the fixed points (melting ice and boiling water) of a mercury thermometer, and point out any precaution that ought to be observed.

A thermometer with an arbitrary scale of equal parts reads 14.6 in melting ice and 237'9 in water boiling under standard pressure. Find the Centigrade temperatures indicated by the readings 971 and 214'0 on this thermometer. (Univ. Coll., Lond., Faculty of Medicine, 1897.)

(6) State what means you would employ to measure (1) a very high temperature, (2) a very small difference of temperature. (Inter. Sci. Hon., Lond., 1894). (See also Ch. XIX.)

CHAPTER II

ERRORS OF A MERCURIAL THERMOMETER, WITH THEIR

CORRECTIONS

(THE reading of this chapter may, if necessary, be postponed till after Chapters III. and IV. have been read.)

Correction to be applied in order to determine the true Boiling-point.-The boiling-point of a liquid varies with the pressure to which it is subjected. The standard pressure adopted is equal to the force exerted by gravity on a vertical column of mercury 76 cm. long, and possessing a sectional area of I sq. cm., the mercury being at the temperature o° C., and being placed at the sea level in latitude 45°.

Fig. 13 represents Fortin's Barometer, the instrument most commonly used for determining the pressure of the atmosphere. A long glass tube is entirely filled with mercury, and then inverted, and placed with its lower extremity in a mercury reservoir R. The height of the surface of the mercury in the tube, above the level of the mercury in the reservoir, gives the barometric height. In order to maintain the surface of the mercury in the reservoir at a constant level, the lower part of the reservoir is made of leather (Fig. 14), and this can be raised or lowered by the aid of a screw, A (Fig. 13), till the mercury surface coincides with the point of an ivory pin P. The graduations on the scale S represent heights above the point of the pin P.

The barometer having been read, the following corrections