Thermometry

Def: The study of thermometers.

oF –

Fahrenheit is a temperature scale named after Daniel Gabriel Fahrenheit (1686–1736), a Germanphysicist who proposed it in 1724.

In this scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point 212 °F, placing the boiling and freezing points of water exactly 180 degrees apart. A degree on the Fahrenheit scale is 1/180th part of interval between the ice point and steam point or boiling point.

Absolute zero is −459.67 °F. The Rankine temperature scale was created to use degrees the same size as those of the Fahrenheit scale, such that a temperature difference of one degree Rankine (1 °R) is the same as a temperature difference of 1 °F, but with absolute zero being 0 °R.

History

According to Fahrenheit himself in an article he wrote in 1724,[1] he determined his scale by reference to three fixed points of temperature. The zero point is determined by placing the thermometer in a mixture of ice, water, and ammonium chloride, a salt. This is a type of frigorific mixture. The mixture automatically stabilizes its temperature at 0 °F. He then put an alcohol or mercury thermometer into the mixture and let the liquid in the thermometer descend to its lowest point. The second point is the 32nd degree found by putting the thermometer in still water as ice is just forming on the surface.[2] His third point, the 96th degree, was the level of the liquid in the thermometer when held in the mouth or under the armpit. Fahrenheit noted that, using this scale, mercury boils at around 600 degrees. Later work by other scientists observed that water boils about 180 degrees higher than the freezing point and decided to redefine the degree slightly to make it exactly 180 degrees higher.[1] It is for this reason that normal body temperature is 98.6 on the revised scale (whereas it was 96 on Fahrenheit's original scale).[3]

A frigorific mixture is a mixture of two or more chemicals that achieve an equilibrium temperature independent of the temperature that the two chemicals started at. The temperature is also relatively independent of the quantities of mixtures as long as significant amounts of each original chemical are present in its pure form.

Liquid water and ice, for example form a frigorific mixture at 0 degrees Celsius or 32 degrees Fahrenheit. A mixture of sodium chloride and ice form a frigorific mixture at -17.8 degrees Celsius or 0 degrees Fahrenheit. Other examples of frigorific mixtures include [1]:

Uses

The most common use of a frigorific mixture is to melt ice. When salt is placed on ice when the ambient temperature is greater than −17.8 °C, then the salt melts some of the ice and the temperature drops to −17.8. Since the mixture is colder than the ambient, heat is absorbed and the temperature rises. This causes the salt to melt more of the ice to drive the temperature down again. The process continues until all of the salt is dissolved in the melted ice. If there is enough salt present, then all of the ice will be melted.

Frigorific mixtures are commonly used in laboratories as a convenient way to generate reference temperatures for calibrating thermometers.

They are also useful for creating cold temperatures when mechanical refrigeration is not available.

Celsius-

The Celsiustemperature scale was previously known as the centigrade scale. The degree Celsius (symbol: °C) can refer to a specific temperature on the Celsius scale as well as serve as a unit increment to indicate a temperature interval (a difference between two temperatures or an uncertainty). "Celsius" is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale two years before his death.

From 1744 until 1954, 0°C on the Celsius scale was defined as the freezing point of water and 100°C was defined as the boiling point of water under a pressure of one standard atmosphere; this close equivalency is taught in schools today. However, the unit "degree Celsius" and the Celsius scale are currently, by international agreement,[1] defined by two different points: absolute zero, and the triple point of VSMOW (specially prepared water). This definition also precisely relates the Celsius scale to the Kelvin scale, which is the SIbase unit of temperature (symbol: K). Absolute zero—the temperature at which no energy remains in a substance—is defined as being precisely 0K and −273.15°C. The triple point of water is defined as being precisely 273.16K and 0.01°C.

History –

In 1742 Swedish Anders Celsius (1701–1744) created a "reversed" version of the modern Celsius temperature scale whereby zero represented the boiling point of water and one hundred represented the freezing point of water. In his paper Observations of two persistent degrees on a thermometer, he recounted his experiments showing that ice's melting point was effectively unaffected by pressure. He also determined with remarkable precision how water's boiling point varied as a function of atmospheric pressure. He proposed that zero on his temperature scale (water's boiling point) would be calibrated at the mean barometric pressure at mean sea level. This pressure is known as one standard atmosphere. (The BIPM's 10th CGPM later defined one standard atmosphere to equal precisely 1,013,250 dynes per cm2 (101.325kPa))

Two years later, that is, 1744—coincident with the death of Anders Celsius— the famous Swedish botanist Carolus Linnaeus (1707–1778) effectively reversed[5] Celsius's scale upon receipt of his first thermometer featuring a scale where zero represented the melting point of ice and 100 represented water's boiling point. His custom-made "linnaeus-thermometer", for use in his greenhouses, was made by Daniel Ekström, Sweden's leading maker of scientific instruments at the time and whose workshop was located in the basement of the Stockholm observatory. As often happened in this age before modern communications, numerous physicists, scientists, and instrument makers are credited with having independently developed this same scale;[6] among them were Pehr Elvius, the secretary of the Royal Swedish Academy of Sciences (which had an instrument workshop) and with whom Linnaeus had been corresponding; Christian of Lyons; Daniel Ekström, the instrument maker; and Mårten Strömer (1707–1770) who had studied astronomy under Anders Celsius.

Kelvin –

The kelvin unit and its scale, by international agreement, are defined by two points: absolute zero, and the triple point of Vienna Standard Mean Ocean Water (VSMOW).[1] This definition also exactly relates the Kelvin scale to the Celsius scale. Absolute zero—the temperature at which nothing could be colder and no thermal energy remains in a substance—is, by definition, exactly 0 K and −273.15 °C. The triple point of water is, by definition, exactly 273.16 K and 0.01 °C. This definition does three things:

  1. It fixes the magnitude of the kelvin unit as being exactly 1 part in 273.16 of the difference between absolute zero and the triple point of water;
  2. It establishes that one kelvin has exactly the same magnitude as a one-degree increment on the Celsius scale; and
  3. It establishes the difference between the two scales’ null points as being exactly 273.15 kelvins (0 K ≡ −273.15 °C and 273.16 K ≡ 0.01 °C). Temperatures in kelvin can be converted to other units per the table at bottom left.

History –

  • 1848:Lord Kelvin (William Thomson), wrote in his paper, On an Absolute Thermometric Scale, of the need for a scale whereby “infinite cold” (absolute zero) was the scale’s null point, and which used the degree Celsius for its unit increment. Thomson calculated that absolute zero was equivalent to −273 °C on the air thermometers of the time.[8] This absolute scale is known today as the Kelvin thermodynamic temperature scale. It’s noteworthy that Thomson’s value of “−273” was actually derived from 0.00366, which was the accepted expansion coefficient of gas per degree Celsius relative to the ice point. The inverse of −0.00366 expressed to five significant digits is −273.22 °C which is remarkably close to the true value of −273.15 °C.

William Thomson's father, Dr. James Thomson, was a teacher of mathematics and engineering at Royal Belfast Academical Institution and the son of a farmer. James Thomson married Margaret Gardner in 1817 and, of their children, four boys and two girls survived infancy. Margaret Thomson died in 1830 when William was only six years old. [2]

William and his elder brother James were tutored at home by their father while the younger boys were tutored by their elder sisters. James was intended to benefit from the major share of his father's encouragement, affection and financial support and was prepared for a fashionable career in engineering. However, James was a sickly youth and proved unsuited to a sequence of failed apprenticeships.

  • In 1832, his father was appointed professor of mathematics at Glasgow and the family relocated there in October 1833. The Thomson children were introduced to a broader cosmopolitan experience than their father's rural upbringing, spending the summer of 1839 in London and, the boys, being tutored in French in Paris. The summer of 1840 was spent in Germany and the Netherlands. Language study was given a high priority