Life Sciences Glossary: Measurement of Temperature and Scales

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Measurement of Temperature and Scales

The temperature of a substance is measured by thermometer. For measurements, three different scales are used. There are Celsius, Fahrenheit and Kelvin (or absolute) scales.

Celsius Scale

This scale was invented by the Swedish astronomer Celsius. On this scale the lower fixed point is taken to be 0o c which is the temperature of pure melting ice and the upper fixed point is marked 100c which is the temperature of boiling water at sea level. This interval between them is divided into 100 equal degrees.

Fahrenheit Scale

This scale was suggested by Fahrenheit and is commonly used for clinical purposes. On this scale the lower fixed point is marked 32F and corresponds to 0c and the upper limit is marked 212f which corresponds to 100c. The conversion table from Celsius to Fahrenheit and vice versa. At -40 both the Celsius and the Fahrenheit scales are equal.

Kelvin Scale

This scale was developed by Lord Kelvin. Unlike the other two scales, this scale is used to measure very low temperatures. This scale has been calibrated on the basis of absolute zero. Absolute zero is the minimum temperature that can theoretically exist in the universe. Absolute zero is minus 273.16c on the Celsius scale. In the Kelvin scale, this has been taken as 0 K (it is not 0K) and 273.16K corresponds to 0C and 373.16K to 100C.

Clinical Thermometer

This thermometer has been specially designed for measuring the temperature of the human body. The normal temperature of the human body is 36.9C or 98.4F. Therefore, this thermometer has a short range of 95F to 110F (0r 35C to 43C) . Another special feature of the thermometer is that there is a constriction in the stem of the capillary tube near the bulb. The constriction prevents the expanded mercury to fall back into the bulb, unless a slight jerk is given to the thermometer. This enables the doctor to read the temperature at leisure and accurately.

Thermal Expansion

When the temperature is increased, two important effects are observed in matter. They are the increase in size and the change of state (melting and boiling) . When matter in any state (solid, liquid or gas) is heated it expands. For a given rise in temperature, gases expand more than liquids and liquids expands more than solids.

Thermal Expansion of Solids

The thermal expansion of solids has both bad and good effects. In order to avoid the bad effects, certain precautions are taken.

For example:

  • Gaps are allowed between two successive rails for the rails to expand. If no gap is provided in summer due to expansion the rails would bend. Similarly, gap is provided in long iron bridges.
  • Telegraph wires are never kept tight. They are given a sag to allow for their contraction in winter.
  • Concrete roads are not laid out in one continuous piece but are laid in small pieces with gaps in between to allow for expansion in summer.

The useful application of thermal expansion of solids are:

  • Iron and steel Tires are tightly fit on cart- wheels by initially heating them
  • Attaching steel plates and girders by riveting. In riveting, the rivets

Are first heated and fixed in the holes of plates to be riveted. Then the ends of the rivet are hammered into heads when the rivet cools, it contracts and pulls the plates together. A thermostat also works on the basis of thermal expansion of solids.

Anomalous Thermal Expansion of Water

  • Liquids on heating, expand uniformly i.e.. , their volume increases linearly with temperature. But water is an exception to this general rule. It shows unusual (or anomalous) behavior. When water at 0C is heated, in the temperature range of 0c to 4c it contracts instead of expanding. On further heating beyond 4c, like other liquids, it starts expanding. Water, therefore, has its minimum volume and maximum density at 4c.
  • The peculiar expansion of water has a significant effect on aquatic life during very cold weather. In cold countries, as the temperature of atmosphere falls, the top layer of a pond or lake cools first. On cooling, it contracts and hence becomes denser than the water below and so sinks. The water from below comes up to take its place. A circulation is thus set up until the entire water of the pond reaches 4c when the temperature of the top layer of water falls below 4c, it expands and becomes less dense and therefore, no longer sinks and ultimately it freezes. Since ice is a poor conductor of heat, it prevents the loss of heat from the pond and helps in keeping the water below at 4c. Thus, the entire pond does not freeze, which allows the aquatic life to survive.

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