We do this by assigning a temperature of 273.16 K to the triple point of water. Of course, we have to calibrate the ideal-gas thermometer itself before we can use it. However, the ideal-gas thermometer is used to calibrate other thermometers. In practice, the ideal-gas thermometer is not as convenient to use as other thermometers-like the mercury-in-glass thermometer. In general, the volume of a given liquid (or solid) substance is not exactly proportional to the volume of a second liquid (or solid) substance over a wide range of temperatures. The (very nearly) direct proportionality of two low-pressure real gas volumes contrasts with what we observe for liquids and solids. Of course, practical problems emerge when we attempt to make such measurements at very high and very low temperatures.
In so far as any gas behaves as an ideal gas at a sufficiently low pressure, any real gas can be used in an ideal gas thermometer and to measure any temperature accurately. When we do so, our device is called the ideal gas thermometer. In principle, we can measure the same temperature using any gas, so long as the constant operating pressure is low enough. This means that we can define temperature in terms of the expansion of any constant-pressure gas that behaves ideally. Moreover, this proportionality is observed for any choice of either gas. To a very good approximation, we find: If we keep the pressures in the thermometer and in some other gaseous system constant at low enough values, both gases behave as ideal gases, and we find that the volumes of the two gases are proportional to each other over any range of temperature. This fact proves to be very useful because of a further experimental observation. That is, our gas-volume measuring device is itself a thermometer. There is a further difficulty with using a liquid as the standard fluid on which to base our temperature measurements: temperatures outside the liquid range of the chosen substance have to be measured in some other way.Įvidently, we can choose to use a gas as the working fluid in our thermometer. That is, if we base our temperature scale on a liquid or solid substance, we observe deviations from Charles’ law. If we make sufficiently accurate measurements, the volume of a gas is not exactly proportional to the volume of any liquid (or solid) that we might choose as the working substance in our thermometer. With sufficiently accurate volume measurements, this occurs to some extent for any choice of the liquid in the thermometer. Careful experiments with such thermometers produce results that deviate from Charles’ law. As we note in Section 2.8, there is a problem with this statement. Our statement of Charles’ law asserts that the volume of a gas is a linear function of the volume of the liquid in our thermometer, and that the same linear function is observed for any gas.
We suppose that this thermometer uses a liquid, and we define an increase in temperature by the increase in the volume of this liquid. In Section 2.2 we suppose that we have a thermometer that we can use to measure the temperature of a gas.
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