A thermometer is a device used to measure temperature of body or object. The measurement of temperature is called as thermometry. The measurement of temperature of any body is based on thermal equilibrium i.e when the temperature of any body or object is to be measured, the measuring device gains temperature equals to that body and then records it.
Different inventions in the field of thermometry had done and all has its own advantages in the fields for which it is made. The first invention of thermometer is credited to Hero of Alexandria. More advanced and accurate devices has been replacing the older devices nowadays.
Types of thermometer according to thermometric substances
There are different types of thermometer according to thermometric substances. Thermometric substances are substance that is used to fill inside thermometer which expands or shows effect on changing temperature. It can be listed as:
1] Liquid Thermometer
The thermometer which uses liquid as thermometric substances is called liquid thermometer. It is based on expansion of liquid on heating. It is of 2 types.
- Mercury thermometer
It uses mercury which expands on heating. On heating the volume of mercury increases and temperature is noted to that point. It measures temperature between range -35°C to 357°C. It is not used to measure very high temperature as it starts vaporizing in case of expanding.
- Alcohol Thermometer
It uses alcohol as thermometric substance. On heating the volume of alcohol increases and temperature is noted to that point. It measures temperature between range -110°C to 76°C.
2] Gas Thermometer
Thermometer which uses gas as thermometric substance are called as gas thermometer. H2, N2 and He2 are used as thermometric substance. It can measure temperature between range -268°C to 1500°C. Gas thermometer are most sensitive, accurate and used to calibrate other thermometer. It is of 2 types.
- Constant volume gas thermometer
In this type of thermometer, volume remains constant whereas pressure changes on changing temperature. It is based on Guy Lussac’s Law (P∝T).
- Constant pressure gas thermometer
In this type of thermometer, pressure remains constant whereas volume changes on changing temperature. It is based on Charles’s Law (V∝T).
3] Resistance Thermometer
It is a type of thermometer which is based on variation of resistance of conductor with temperature. It is also called as platinum resistance thermometer due to use of platinum in it. It is based on principal of change in electrical resistance with rise in temperature or Kirchoff’s or Wheatstone bridge principle. It measures temperature range between -200°C to 1200°C. It is used to measure temperature inside motor engine.
4] Thermoelectric/Thermocouple Thermometer
It is based on variation of thermo emf in thermocouple with temperature. It is based on principle of Seebeck effect. It’s range of temperature is between -200°C to 1600°C. It is used to measure rapidly changing temperature.
It is based on principle of Stefan’s law. It can measure temperature of source from any distance. It is used to measure very high temperature. Range of temperature is between -800°C to 4000°C.
6] Vapour pressure thermometer
It is based on saturated vapour pressure of liquid varying with temperature. It is used to measure very low temperature ranging between 0.17 K to 122 K.
7] Magnetic thermometer
It is based on variation of magnetic susceptibility with temperature. It is based on principle of Curie law. Ferromagnetic substance is used as thermometric substance in it. It is used to measure temperature below 1 Kelvin.
Types of thermometer according to uses
There are different types of thermometer according to their use in different fields. Some of them are:
1] Medical or Clinical thermometer
These types of thermometer are used in hospital and clinic. It has mercury as thermometric substance which is filled inside it. It has cylindrical bulb to provide maximum surface for transferring heat. It is used to measure temperature of patient and recording degree of fever.
2] Laboratory thermometer
It is used in laboratory activities purposes. Its plays important role in measuring temperature of different laboratory elements like room temperature, temperature of water, solutions, solids and during different intervals of experiment. Nowdays, more advanced technology are replacing the old devices for better, reliable and for faster work. Digital and software operated devices are used which shows temperature of each minute once setup in the lab.
3] Digital Thermometer
This type of thermometer uses numeric values and display screen on which it shows temperature. They are very easy to use and advanced then older thermometers. They are also used for clinical purposes.
4] Infrared Ear Thermometer
It uses infrared rays to measure temperature of the inner ear. It is developed to measure temperature of inner organs like ear because other thermometer cannot used for this purpose. When it is closely placed inside ear, infrared radiation radiating from tympanic membrane falls on infrared detector of this device and temperature is calculated and shown by the device.
There are different types of temperature scale in which the same temperature is expressed in different measurement systems. Kelvin (K) is the S.I unit of temperature. There are 5 main scales on which temperature is expressed. They are:
It is commonly used scale in which temperature is measured. It is symbolically represented as C. It is also sometimes called as centigrade scale. It lower fixed point is 0°C whereas upper fixed point is 100°C. It has total 100 divisions in which each division is equal to 1 degree centigrade. In this scale water freezes at 0°C and boils at 100°C at normal temperature and pressure (NTP) or room temperature.
It was created by German Scientist Daniel Gabriel Fahrenheit. It is symbolically represented as F. It’s lower fixed point is 32°F whereas upper fixed point is 212°F. It has 180 divisions in which each division is equal to 1°F.
In this scale water freezes at 32°F and boils at C at 212°F at normal temperature and pressure (NTP) or room temperature.
It is called as absolute scale or standard scale. It is symbolically represented as K and it do not uses degree symbol. It’s lower fixed point is 273K whereas upper fixed point is 373K. It has 100 divisions in which each division is equal to 1K. It was designed to measure absolute zero temperature. Absolute zero temperature is 0K or -273.15°C. In this scale water freezes at 273.15K and boils at 373.15K at normal temperature and pressure (NTP).
This scale is named after French entomologist and writer Rene Antoine Ferchault de Reaumer. It is symbolically represented as Re. It’s lower fixed point is 0°Re whereas upper fixed point is 80°Re. It has 80 divisions in which each division is equal to 1°Re.
In this scale water freezes at 0°Re and boils at 80°Re at normal temperature and pressure (NTP).
This scale was invented by Scottish Scientist William John Rankine in 19th century. It is symbolically represented as Ra. It’s lower fixed point is 492°Ra whereas upper fixed point is 672°Ra. It has 180 divisions in which each division is equal to 1°Ra.
In this scale water freezes at 491.67°Ra and boils at 671.64°Ra.
Temperature scales formula
Any temperature can be converted into any measuring scale. On conversion of temperature from one scale to others, quantity remains the same only the method of expressing differs. Temperature scale formula can be derived by subtracting lower fixed point and then dividing it by total nos. of division in that scale.
Change in temperature can be related as:
Note: Change in temperature in Celsius and Kelvin scale are same because they have same nos. of divisions.
Some Quick Facts
►Celsius and Fahrenheit scale gives common reading at -40°C (-40°C = -40°F).
►Celsius and Reaumer scale gives common reading at 0°C (0°C = 0°Re).
►Fahrenheit and Reaumer scale gives common reading at -25.6°F (-25.6°F = -25.6°Re).
►Celsius and Kelvin scale, Fahrenheit and Rankine scale never give common reading.
Frequently asked questions on thermometer
How do Thermometer measures temperature?
Ans: Thermometers measures temperature on the basis of thermal equilibrium. When a thermometer is put inside body to measure temperature, it gains heat from that body which in terns cause expansion of the liquid used inside the device. The expansion occurs until equilibrium is maintained on both side. Thus, the volume up to which the liquid expanded is the temperature of that body.
Which is the most sensitive thermometer?
Ans: Gas thermometer are most sensitive thermometer as their expansion is very rapid as compared to other thermometer.
Which thermometer is used to measure temperature of sun?
Ans: Temperature of surface of sun is measured by using pyrometer.
What is meant by calibration of thermometer?
Ans: Calibration is used to graduate newly discovered/manufacture thermometer to find their accuracy, to mark its lower fixed point and upper fixed point with the help of standard known thermometer or other reliable techniques.
Which is best to used in thermometer spherical or cylindrical bulb?
Ans: Rate of transfer of heat is directly proportional to the available surface area. Larger is the surface area faster is the transfer of heat and more accuracy the reading of thermometer. As cylindrical bulb has more available surface area than spherical bulb, thermometer with cylindrical bulb shows more accurate temperature and preferred.