Modern Engineering Thermodynamics

8 CHAPTER 1: The Beginning
NURSERY RHYMES AND UNITS Continued
today is in the term jackpot. Coincidentally, the next larger unit size, the gill (1 gill = 2 jacks), also fell into disuse. The
political meaning of the following popular Mother Goose rhyme should now become clear (Figure 1.7):
Jack and Gill went up a hill to fetch a pail of water.
Jack fell down and broke his crown and Gill came tumbling after.
The Jack and Gill in this rhyme are not really a little boy and girl, they are the old units of volume measure. Jack fell down refers to the
fall of the jack from popular usage as a result of the tax imposed by the crown, Charles I. The phrase and Gill came tumbling after refers
to the subsequent decline in the use to the gill unit of volume measure. The “real” jack and gill of this rhyme are shown in Figure 1.8.
Jack
Gill
FIGURE 1.7
Jack and Jill.
FIGURE 1.8
The real jack and gill.
CRITICAL THINKING
What other Mother Goose rhymes or children’s songs are not what they seem?
1.6 TEMPERATURE UNITS
The development of a temperature unit of measure came late in the history of science. The problem with early
temperature scales is that all of them were empirical, and their readings often depended on the material (usually
a liquid or a gas) used to indicate the temperature change. In a liquid-in-glass thermometer, the difference
between the coefficient of thermal expansion of the liquid and the glass causes the liquid to change height
when the temperature changes. If the coefficient of thermal expansion depends in some way on temperature,
then an accurate thermometer cannot be made simply by defining two fixed (calibration) points and subdividing
the difference between these two points into a uniform number of degrees. Unfortunately, the coefficients of
thermal expansion of all liquids depend to some extent on temperature; consequently, the two-fixed-point
method of defining a temperature scale is inherently prone to this type of measurement error.
In 1848, William Thomson (1824–1907), later to become Lord Kelvin, developed a thermodynamic absolute
temperature scale that was independent of the measuring material. He was further able to show that his thermodynamic
absolute temperature scale was identical to the ideal gas absolute temperature scale developed earlier,
and therefore an ideal gas thermometer could be calibrated to measure thermodynamic absolute temperatures.
Thereafter, the absolute Celsius temperature scale was named the Kelvin scale in his honor. Because it was a real
thermodynamic absolute temperature scale, it could be constructed from a single fixed calibration point once
the degree size had been chosen. The triple point of water (0.01°C or 273.16 K) was selected as the fixed point.