What is a thermometer ??

Introduction of thermometer

 

The instrument which is used to measure temperature is called thermometer. Thermometers are based on the principle that matter expands on heating and contracts on cooling.

Construction of a  thermometer

The structure of a thermometer can be described  under the following headings.

a) Capillary tube: The thermometer consists of a very fine glass tube having a very small bore called capillary tube.

b) Stem: The capillary tube is protected by a thick glass tube called stem.

c) Bulb : At one end of the capillary tube , a very thin  walled glass is provided, called bulb. It is filled with thermometric liquid (mercury or alcohol).

d) Standard Points: Stream point (i.e  upper fixed point ) and ice point ( i.e. lower fixed point ) are the  two standard points fixed on the stem of a thermometer to make a thermometer scale. It is called calibration. The temperature, at which water boils at standard atmospheric pressure is called the upper fixed point and the temperature at which water freezes at standard atmospheric pressure is called the lower fixed point.

Temperature Scales

Temperature is measured generally in three different scales.

i) Celsius or Centigrade Scale

It is one of the most widely used temperature scales. This scale was introduced by a Swedish astronomer Anders Celsius and is known after his name. In this scale, there are 100 divisions between the upper fixed point (i.e,100 degree Celsius ) and lower fixed point ( i.e. 0 degree Celsius) . Each division on this scale is called one degree centigrade or Celsius.

ii) Fahrenheit Scale

This scale was introduced by German physicist Daniel Gabriel Fahrenheit and is known after his name. There are 180 divisions between the upper fixed point (i.e, 212 degree F) and lower fixed point ( i.e. 32degree F) in this scale. Each division on this scale is called one degree Fahrenheit.

iii) kelvin Scale 

Kelvin temperature scale is named after the British physicist William Thomson, Baron Kelvin. This scale of temperature is used in the scientific research. In this scale, there are 100 divisions between the  upper fixed point ( i.e. 373 K ) and lower fixed point (i.e 273 K). Each division on this scale is called one kelvin.

Types of thermometer based on thermometric liquids

On the basis of the thermometric liquid used , there are two types of thermometers . They are mercury thermometer and alcohol thermometer,

a) Mercury  Thermometer

The thermometer in which we use mercury as a thermometric liquid is called mercury thermometer.

Advantage of mercury as a thermometric liquid

1. Mercury is a good conductor of heat .It needs very little heat to expand.
2. It does not wet the glass. Thus it helps in recording accurate temperature.
3. Expansion of mercury is uniform over a wide range of temperature.
4. It is opaque and silvery in color. Thus , it can be clearly seen in the capillary tube.
5. Boiling temperature of mercury  is 357 degree C. So, a temperature as high as 357 degree Celsius can be recorded with the help of a mercury thermometer.

Disadvantage of a mercury thermometer

1. Mercury is a very expensive liquid metal.
2. Freezing temperature of mercury is -39degreeC . So, a mercury thermometer can record only up to -39degree C temperature. 
3. Mercury is a poisonous substances. It becomes dangerous if the thermometer tube is broken.

b) Alcohol Thermometer 

The thermometer in which we use alcohol as a thermometric liquid is called alcohol thermometer.

Advantage of alcohol thermometer

1.  Melting point of alcohol is -117degree C. So, an alcohol thermometer can record a temperature as low as -117degree C. Alcohol thermometer can record the temperature in Arctic and Antarctic region.
2. Alcohol expands more than mercury for the same rise or fall in temperature . Thus, an alcohol thermometer can measure temperature more accurately than mercury.

Disadvantage of an alcohol thermometer:

1. Alcohol should be brightly colored to use it in thermometer.
2. Boiling temperature of alcohol is 78 degree C . So, alcohol thermometer cannot measure high temperatures.

Types of thermometers on the basis of uses

On the basis of uses, there are different types of thermometers. Among them, clinical thermometer , laboratory thermometer and maximum and minimum thermometers are described here.

a) Clinical Thermometer 

A thermometer which is used to measure our body temperature is called clinical thermometer. It is also called doctor's thermometer.

i) Clinical  thermometer containing mercury: 

Mercury is used in this clinical thermometer. It has a prismatic shape which helps to magnify the mercury thread called kink. It prevents the backflow of mercury . Clinical thermometer is jerked before it use, As a result, mercury returns into the bulb. It ranges from35 degree Celsius to 43degree Celsius or 94 degree F to 108 degree F.

ii) Digital Thermometer:

Digital thermometer is a mercury free thermometer which contains thermistor inside the tip to measure temperature. Nowadays, digital clinical thermometers are used widely to measure our body temperature. They provide highly accurate result of the  body temperature. It reads temperature when the built-in sensor is kept in contact with the human body. They are equipped with beep alarm and memory function. This helps to record a wide range of temperature.

A digital thermometer has following advantages over the mercury-used clinical thermometer;

1. It is more accurate than bulb thermometer
2. It shows the temperature directly into digits.
3. It is toxic-free thermometer. If a mercury thermometer breaks than the disposal of mercury creates a problem.

c) Laboratory Thermometer

A thermometer which is used to measure temperature of the different substances in the laboratory is called laboratory thermometer. A laboratory thermometer does not have kink. A common laboratory thermometer can measure temperature from -10 degree Celsius to 110 degree Celsius. 

d) Maximum and Minimum Thermometer 

The thermometer which is used to measure the highest and lowest temperature during a day is called a maximum and a minimum thermometer. 

Construction 

This thermometer contains a bulb A which contains alcohol and a second bulb, which is nearly full of alcohol with a vacuum at its top. A U-shape tube 'C' containing mercury, connects the two bulbs. 

Working 

When the temperature rises, the alcohol in bulb A expands and pushes the mercury thread up in the right-hand side of the thermometer. The vacuum in another column provides the space for expansion. Mercury thread pushes the steel index. The steel pointer X marks the highest temperature at which the mercury thread has stopped. When the alcohol in bulb A contracts and the mercury thread moves up in the left-hand side pushing the steel index Y, the index Y remains in a position corresponding to the lowest temperature. The thermometer is reset by dragging the steel indicates back to their original position with the help of a magnet.