CHAPTER 11 : TEMPERATURE AND THERMOMETERS

We are aware of notions of 'hot' and 'cold' through our sense of touch; but this is a subjective and misleading notion. For, if the left hand of a person is kept immersed in cold water and the right hand in hot water for a sufficiently long period of time (few minutes) and then both hands are immersed in luke warm water the resulting sense of perception of the left hand is that the 'luke warm' water is hot and that of right hand is that the same 'luke warm' is cold.

The first step towards an objective measure of temperature is to determine the unique property of some substance like mercury, to which temperature can be uniformly co-related. Mercury, in the form of a thread in a narrow bore of a glass tube expands uniformly over the same interval of change in its temperature. This thermometric property of mercury is used to define three temperature scales, Celsius, Fahrenheit and Roemer. In each of these scales the melting point of ice and the boiling point of water, under 1 atmospheric pressure, are used as standard references.

Celsius Scale

The melting point of ice is taken as 0⁰C and the boiling point of water is taken as 100⁰C, and the interval between these two reference levels is divided into 100 equal divisions, each division is called 1⁰C   (1⁰ Centigrade or 1⁰ Celsius).

Fahrenheit Scale

The melting point of ice is taken as 32⁰F and the boiling point of water is taken as 212⁰F; the interval between these levels is divided into 180 equal parts, each part is called 1⁰F (1⁰Fahrenheit).

Roemer Scale

The melting point of ice is taken as 0⁰R and the boiling point of water is taken as 80⁰R; the interval between these levels is divided into 80 equal parts, each part is called 1⁰R (1⁰ Roemer).

The temperature from one scale to the other can be converted by using the relation,

Now a days Celsius scale or Kelvin scale is used in scientific work; Fahrenheit scale is used largely in clinical thermometers; whereas Roemer scale is not used anywhere.

We shall take up Kelvin scale in Chapter 12 and again in Chapter 15.

Solved Problem

1. If the temperature of an object is 50⁰C, then what will this temperature be in ⁰F and ⁰R ?

Solution :

\ Centrigrade temp 500C is 1220 F or 400 R.

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Index

Heat and Thermodynamics Chapter 11

Chapter 12