the zeroth law of thermodynamics states that if two bodies are in thermal
equilibrium with a third body, they are also in thermal equilibrium with
each other. it may seem silly that such an obvious fact is called one of the
basic laws of thermodynamics. however, it cannot be concluded from the
other laws of thermodynamics, and it serves as a basis for the validity of
temperature measurement. by replacing the third body with a thermometer,
the zeroth law can be restated as two bodies are in thermal equilibrium if
both have the same temperature reading even if they are not in contact.
the zeroth law was first formulated and labeled by r. h. fowler in 1931.
as the name suggests, its value as a fundamental physical principle was recognized
more than half a century after the formulation of the first and the
second laws of thermodynamics. it was named the zeroth law since it
should have preceded the first and the second laws of thermodynamics.
should be noted that the absolute gas temperature scale is not a thermodynamic
temperature scale, since it cannot be used at very low temperatures
(due to condensation) and at very high temperatures (due to dissociation and
ionization). however, absolute gas temperature is identical to the thermodynamic
temperature in the temperature range in which the gas thermometer
can be used, and thus we can view the thermodynamic temperature scale at
this point as an absolute gas temperature scale that utilizes an “ideal” or
“imaginary” gas that always acts as a low-pressure gas regardless of the
temperature. if such a gas thermometer existed, it would read zero kelvin at
absolute zero pressure, which corresponds to 273.15°c on the celsius.
the international temperature scale of 1990, which supersedes the international
practical temperature scale of 1968 (ipts-68), 1948 (itps-48),
and 1927 (its-27), was adopted by the international committee of weights
and measures at its meeting in 1989 at the request of the eighteenth general
conference on weights and measures. the its-90 is similar to its predecessors
except that it is more refined with updatingd values of fixed
temperatures, has an extended range, and conforms more closely to the
thermodynamic temperature scale. on this scale, the unit of thermodynamic
temperature t is again the kelvin (k), defined as the fraction
1/273.16 of the thermodynamic temperature of the triple point of water,
which is sole defining fixed point of both the its-90 and the kelvin scale
and is the most important thermometric fixed point used in the calibration
of thermometers to its-90.
the unit of celsius temperature is the degree celsius (°c), which is by
definition equal in magnitude to the kelvin (k). a temperature difference
may be expressed in kelvins or degrees celsius. the ice point remains the
same at 0°c (273.15°c) in both its-90 and itps-68, but the steam point
is 99.975°c in its-90 (with an uncertainly of 0.005°c) whereas it was
100.000°c in ipts-68. the change is due to precise measurements made by
gas thermometry by paying particular attention to the effect of sorption (the
impurities in a gas absorbed by the walls of the bulb at the reference temperature
being desorbed at higher temperatures, causing the measured gas
pressure to increase).
the its-90 extends upward from 0.65 k to the highest temperature practically
measurable in terms of the planck radiation law using monochromatic
radiation. it is based on specifying definite temperature values on a
number of fixed and easily reproducible points to serve as benchmarks and
expressing the variation of temperature in a number of ranges and subranges
in functional form.
in its-90, the temperature scale is considered in four ranges. in the
range of 0.65 to 5 k, the temperature scale is defined in terms of the vapor
pressure—temperature relations for 3he and 4he. between 3 and 24.5561 k
(the triple point of neon), it is defined by means of a properly calibrated
helium gas thermometer. from 13.8033 k (the triple point of hydrogen) to
1234.93 k (the freezing point of silver), it is defined by means of platinum
resistance thermometers calibrated at specified sets of defining fixed
points. above 1234.93 k, it is defined in terms