TEMPERATURE SCALE

Before going into details of measurement of temperature it is essential to fix its unit. The most common unit used today is the Celsius or centigrade scale. Fahrenheit scale is also used in several countries, For example, in the United States. The mostly used unit in scientific work in the Absolute or Kelvin scale. Ranking scale is not used now.

Lower and upper fixed points in both Celsius and Fahrenheit scales are chosen as freezing and boiling points of water respectively at standard pressure. On the Celsius scale, the frizzing point of water is chosen to be 0ºc and the boiling point of water is 100ºc. On the Fahrenheit scale, the frizzing point is taken at 32ºf and the boiling point at 212ºf. A practical thermometer is calibrated by placing it in a carefully prepared environment at each of two temperature are marking the position of the mercury. The distance between lower fixed point and upper fixed point is divided into 100 equal parts in the Celsius scale where as the same distance is divided into 180 equal parts in the Fahrenheit scale.

THE LIQUID THERMOMETER

As it has mentioned before that the basic of thermometer practically used is based on the principal of expansion or contraction of liquid (change into volume) due to heating or cooling of it respectively. A proper choice of liquid is always essential. Those liquid which is used a thermometer construction called Thermometric liquid and must have following basic character tics

CHARACTER TICS OF THERMOMETRIC LIQUID

1.   It must have low specific heat, so that it absorbs small amount of heat from the body whose temperature is being measured.

2.   It must be good conductor of heat so that it can take heat quickly from a body.

3.   It must be easily visible in the capillary tube.

4.   The liquid should not wet the wall of the capillary tube.

5.   The liquid must have uniform expansively (coefficient of expansion) over wide range of temperature.

6.   The freezing point and baling point of the liquid must be appropriate, so that it remains as liquid in the range of measurement.