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Temperature and Its Measurement

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Temperature and Its Measurement - Lesson Summary


Temperature is a measure of the degree of hotness of a body.  The SI unit of temperature is kelvin (K). But temperature is also measured in celcius (°C) or fahrenheit (°F) scales. The instrument used to measure temperature is called a thermometer. 


A thermometer is used to measure the temperature of an object. Galileo invented a rudimentary water thermometer in 1593. He called this device a thermoscope.  However, this form was ineffective as water freezed at low temperatures.  In 1714, Gabriel Fahrenheit invented the modern mercury thermometer. A thermometer consists of a long, narrow, uniform glass tube called the stem. The scales in which the temperature is measured are marked on the stem. At the end of the stem there is a small bulb which contains mercury in it.  A capillary tube is inside the glass stem in which mercury expands when the bulb is kept in contact with a hot body. Mercury is toxic, and it is very difficult to dispose it when the thermometer breaks. So, nowadays digital thermometers are used to measure the temperature, as they do not contain mercury.

Types of Thermometers

There are different types of thermometers for measuring the temperatures of different things like air, our bodies, food and many other things. There are clinical thermometers, laboratory thermometers, Galileo thermometers, maximum-minimum thermometers and digital remote thermometers. Among these, the commonly used thermometers are clinical thermometers and laboratory thermometers.

Clinical Thermometer

These thermometers are used to measure the temperature of the human body, at home, clinics and hospitals. All clinical thermometers have a kink that prevents the mercury from flowing back into the bulb when the thermometer is taken out of the patien's mouth, so that the temperature can be noted conveniently. There are temperature scales on either side of the mercury thread, one in Celsius scale and the other in Fahrenheit scale. Since the Fahrenheit scale is more sensitive than the Celsius scale, body temperature is measured in °F only. A clinical thermometer indicates temperatures from a minimum of 35 °C or 94 °F to a maximum of 42 °C or 108 °F.

Precautions to be Followed While Using a Clinical Thermometer

     •  The thermometer should be washed before and after use, preferably with an antiseptic solution.
     •  Jerk the thermometer a few times to bring the level of the mercury down.
     •  Before use, the mercury level should be below 35 °C or 94 °F.
     •  Do not hold the thermometer by its bulb.
     •  Keep the mercury level along your line of sight and then take the reading.
     •  Handle the thermometer with care. If it hits against some hard object, it may break.
     •  Do not place the thermometer in a hot flame or in the hot sun.

Laboratory Thermometers
Lab thermometers are used to measure the temperature in school and other laboratories for scientific research. They are also used in the industry as they can measure temperatures higher than what clinical thermometers can record. The stem and the bulb of a lab thermometer are longer when compared to that of a clinical thermometer and there is no kink in the lab thermometer. A laboratory thermometer has only the Celsius scale ranging from −10 °C to 110 °C.

Precautions to be Followed While Using a Laboratory Thermometer
     •  Do not tilt the thermometer while measuring the temperature. Place it upright.
     •  Note the reading only when the bulb has been surrounded by the substance from all sides.

Conversion between Celcius and Fahrenheit Temperatures
The Celsius and Fahrenheit scales are related as follows.

  C 5 = F - 32 9 where C and F are the temperatures measured in Celcius and Fahrenheit scales, respectively. The relation between Celcius and Kelvin temperatures is given by

K = C + 273, where K is the temperature in kelvin.


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