### Robert A. Paselk Scientific Instrument Museum

From: A. Wilmer Duff. A Text-Book of Physics 5th ed. P. Blakiston's Son & Co. Philadelphia (1921) pp. 202-205

THERMOMETRY

263. Standard Scale of Temperature.--We shall throughout use the term temperature to mean a quantity which we are now to define and which can be measured for any body at any time. Differences of temperature are to agree with our ordinary ideas of difference of hotness or coldness, so far as the two can be compared. The scale of temperature which we shall adopt is the international legal standard and is based upon the effect of increase in hotness upon the pressure of hydrogen. Changes of temperature are defined as being proportional to the corresponding changes of pressure in a constant mass of hydrogen confined at constant volume. This is called the hydrogen constant volume scale. To measure the temperature of a body, for example, of a mass of water, the vessel containing the hydrogen would be held in the water and the pressure of the hydrogen measured. But before temperature can be expressed as a number, we must have a unit in which to express it and we must also agree on a reference point or "zero" from which it is to be measured. The ordinary zero called the "ice-point," is the temperature of a mixture of pure ice and water when the pressure on the water surface is 1 atmosphere, while the degree is fixed by adopting a second standard point, the "steam-point," or the temperature of boiling water when the pressure is 1 atmosphere, which is specified as +100° or 100° above zero. The degree is then such a change in temperature as will produce 1/100 the change in pressure which is observed when the hydrogen is heated from the ice-point to the steam-point. These specifications define the Centigrade zero and Centigrade degree, which are universally used in scientific work.

A thermometer is an instrument for measuring temperature according to some definite scale. A constant volume gas thermometer is an apparatus for measuring temperature by the variation in pressure of a gas confined at constant or nearly constant volume. If the gas used is hydrogen the thermometer gives at once standard temperature; with other gases it must be calibrated in terms of the standard. Such an arrangement is shown diagrammatically in Fig. 170, and consists essentially of a bulb of glass, glazed porcelain, fused quartz, platinum or platinum-iridium (according to the temperature range over which it is to be used), connected by a capillary tube to a mercury pressure-gauge such as the open manometer shown. The pressure of the confined gas can be measured by reading the difference in level of the two mercury columns and adding to this the atmospheric pressure as determined by a barometer.

## . . .

It must be understood that the choice of a thermometric property (in this case the pressure of hydrogen) is entirely independent of the choice of numerical scale, i.e., zero and size of degree; the Centigrade or Fahrenheit numerical scale can each be applied to any other thermometric property desired.

It is found that the change in pressure (volume constant) of 1 hydrogen for 1°C. as above defined is very closely 1/273.0 of the pressure at O°C.; hence if the same scale of temperature were carried below zero Centigrade (Fig. 171) the pressure would be reduced to zero at a temperature of about -273.0°C. This is called the absolute zero of the hydrogen constant volume scale, and, according to the ideas of the kinetic theory of gases, it corresponds to a state of zero molecular velocity, since pressure is due to the impact of moving molecules.This temperature could not, however, be measured with the hydrogen thermometer, because, as we shall see, the gas would become liquid before this point was reached. We shall use T to represent temperatures measured from absolute zero on the hydrogen scale, called absolute temperatures. In order to give at once some idea of the known range of temperatures on the centigrade hydrogen scale it may be noted that:

-273.0° = absolute zero.
-271.3° = lowest temperature ever measured.
-190" = temperature of liquid air under 1 atmosphere pressure.
-80° = lowest recorded natural temperature.
0° = melting-point of ice.
100° = boiling point of water under 1 atmosphere pressure.
700° = "dull red" heat for most solids.
1400° = "white heat" for most solids.
3800° = about the temperature of the electric are.
6000°-7000° = Sun's temperature.

Index of Instruments

## Museum Home

Instrument Literature

HSTC (1921-34)
HSC (1935-1953)
HSC (1954-1973)