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- From: A. Wilmer Duff. A Text-Book of Physics
5th ed. P. Blakiston's Son & Co. Philadelphia (1921) pp.
202-205
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- Copyright © 1998 Richard A. Paselk
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- THERMOMETRY
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- 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.
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- 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.
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- 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:
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- -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.
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- © R. Paselk
- Last modified 22 July 2000
