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Temperature is the thermal condition of a body which determines the flow of heat from it to another body while coming in the contact. In a simple language, the temperature is the measure of hotness or coldness of a body. It is the average kinetic energy of the molecules contained by the body.

Temperature of a body can be measured by using an instrument called thermometer. It works on the principle: a body expands on heating and contracts on cooling. It is based on the property of thermal expansion of liquids. Generally, the liquid is used in a thermometer. The thermometer, which includes mercury or alcohol as thermometric liquids, is known as liquid thermometer. Normal temperature of the human body is 37⁰C i.e. 98.6⁰F. The SI unit of temperature is Kelvin.

Thermometric liquid

The liquid used in thermometer is called thermometric liquid. Mercury and alcohol are examples of thermometric liquid.

1. Mercury
Mercury is a thermometric liquid that remains in the liquid state in quite a wide range of temperature because it freezes at -39⁰C and boils at 357⁰C. So, mercury cannot be used to measure the temperature below -39⁰C as it freezes below that temperature. Therefore, we cannot use mercury thermometer to measure the temperature in very cold regions.

2. Alcohol
Alcohol is also a thermometric liquid that freezes below the temperature -117⁰C and boils at 78⁰C. So, alcohol remains liquid at a very low temperature but not at a high temperature. Therefore, an alcohol thermometer is used for measuring the temperature at very cold regions.

Temperature Scale

Temperature is marked in Centigrade scale, Fahrenheit scale and Kelvin scale.

Under centigrade scale, the lower fixed point is 0⁰C and the upper fixed point is 100⁰C. The interval between these two fixed points is divided into 100 equal parts. Each part of division is called one-degree centigrade i.e. denoted as 1⁰C.

2. Fahrenheit Scale
Under Fahrenheit scale, the lower fixed point is 32⁰F and the upper fixed point is 212⁰F. The interval between these two fixed points is divided into 180 equal parts. Each part of division is called one Fahrenheit i.e. denoted as 1⁰F.

3. Kelvin Scale
Under Kelvin scale, the lower fixed point is 273 K and the upper fixed point is 373 K. The interval between these two fixed points is divided into 100 equal parts. Each part of division is called one Kelvin i.e. denoted as 1 K.

Relation between Different Temperature Scales

To find the relation between these temperature scales, the freezing point in each scale is made equal and when the similar freezing point is divided by the difference between the lower fixed point and the upper fixed point, they give the same value of temperature.

Symbolically,

$$\frac {C-10}{100}$$ = $$\frac {F-32}{180}$$ = $$\frac {K-273}{100}$$

Types of Thermometer

There are different types of thermometers in practice. Some of the liquid thermometers are: Clinical thermometer, simple thermometer and maximum-minimum thermometer.

1. Clinical Thermometer
A clinical thermometer is used to measure the temperature of the human body. It is also called the doctors’ thermometer. It has less range of scale i.e. 35⁰C to 42⁰C or 94⁰F to 108⁰F and is smaller in size compared to other thermometers. It can be found in different types of oral, armpit, rectal or ear. It may be mercury or digital.

In a clinical thermometer, there is a bulb filled with mercury at its one end and a glass stem marked with a scale. The bulb has a thin wall while the prismatic stem has a thick wall with a very fine bore known as the capillary. There is a bent narrow part above the bulb in capillary which is known as constriction. It blocks the backflow of the raised mercury level.

To measure the temperature of the body, the bulb of the thermometer is kept in contact with the body for one to two minutes, the heat present in the body makes the mercury in the bulb to expand and the level of mercury rises up in the capillary. The scale shows the rating of the body temperature. For further use of the thermometer, it is jerked to reset the level of mercury below the normal temperature.

2. Laboratory Thermometer
A laboratory thermometer is a simple thermometer which is similar to a clinical thermometer in structure and working method. But constriction is absent in its capillary. It is used to measure the temperature of materials. Usually, it is longer than the clinical thermometer and it has more range of scale i.e. 10⁰C to 110⁰C.

To measure the temperature of any substance, the bulb of the thermometer is kept in contact with the substance. The change in the level of mercury in capillary shows the temperature of the substance.

3. Digital Thermometer
Digital thermometer is an electronic thermometer having a wide range of scale on LCD display. The thermometer has a thermistor at its one end equipped with a beep alarm and a memory function. There is a cap at the top of the thermometer which is made up of thick, hard and smooth plastic which protects the battery kept under it.

To measure the temperature of the body using digital thermometer, it is kept in contact with the body and the temperature is displayed on the screen.

In these present days, an infrared sensor is also used to make non- contact electronic thermometer.

4. Maximum- Minimum Thermometer
A maximum and minimum thermometer is a device that records the highest and lowest temperatures reached over a certain time period or in a day at a certain place. It is made up of U- shaped parallel glass tubes filled with mercury having one side registered with minimum temperature and the other with maximum temperature, and two bulbs. The mercury of one column goes up with the increase in temperature and the mercury of other column goes down with the decrease in the temperature. They work in opposite directions. The minimum and maximum columns above the mercury are filled with a clear liquid or alcohol that expands or contracts with the change in temperature of the environment.

When the temperature of the environment rises up, the alcohol expands that pushes the mercury up in the maximum column and also pushes the level of mercury down in the minimum column. When the temperature of the environment decreases, the alcohol contracts that pulls the mercury of the minimum column causing the fall of mercury in the maximum column of the U- shaped thermometer. The alcohol pushes the mercury in both tubes to record certain temperature readings, what are marked with steel indexes or markers in the tubes that are held in place by small springs. The indexes are located on the surface of the mercury and move when the mercury is pushed by the alcohol. When the temperature reaches their minimum and maximum, the steel indices remain in place allowing the readings of both minimum and maximum temperatures simultaneously without constant monitoring of the thermometer. The steel indexes can be returned to the surface of the mercury by using small magnets.

1. Temperature is the degree of hotness or coldness of a body.
2. The SI unit of temperature is Kelvin (K).
3. It is measured by using thermometer. Thermometers are of many types: simple, clinical, pyrometers, digital, etc.
4. A clinical thermometer is used to measure the temperature of the human body.
5. A laboratory thermometer is a simple thermometer which is similar to a clinical thermometer in structure and working method.
6. Digital thermometer is an electronic thermometer having a wide range of scale on LCD display.
7. A maximum and minimum thermometer is a device that records the highest and lowest temperatures reached over a certain time period or in a day at a certain place.
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#### Click on the questions below to reveal the answers

Temperature is a measure of hotness or coldness of the body. It is the thermal condition of the body which determines the flow of heat from it to another body in its contact.

A thermometer is a device which is used for measuring the temperature of a body. It works on the principle that "when a body is heated, it expands and it contracts on cooling".

The thermometer in which alcohol or mercury is used as a thermometric liquid is known as liquid thermometer.

A clinical thermometer is a thermometer which is used to measure the temperature of the human body. It is also known as daddy thermometer and doctors' thermometer.

A maximum and minimum thermometer is a device that records the highest and the lowest temperatures reached over a certain time period or in a day of a given area.

Mercury cannot be used to measure temperature in very cold regions as it freezes below the temperature of -39°C.

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• ### A ______ thermometer is used to measure the temperature of human body.

laboratory
special
all of them
clinical
maximum and minimum
none of them
• ### A ______ thermometer is also known as simple thermometer.

all of them
maximum and minimum
none of them
laboratory
digital
clinical
• ### A ______ thermometer is an electronic thermometer.

digital
all of them
maximum and minimum
none of them
laboratory
clinical

joule
degree
kelvin
kilometer
grams
dyne

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