## Thermometer

Subject: Science

#### Overview

A device, which measures the temperature of a body, is called thermometer. A mercury thermometer is used to measure the temperature of extreme hot places. An alcohol thermometer is used to measure the temperature of extreme cold places. This note provides us with an information about thermometer and its uses.
##### Thermometer

A device, which measures the temperature of a body, is called thermometer. Three temperature scales are used on thermometers: the Celsius, the Fahrenheit, and the Kelvin scale. Most liquid thermometers use mercury or alcohol as thermometric substances. The relationship between the reading on the temperature scales is given by the relation:

Precautions while using thermometer are:

1. The bulb of the thermometer should come in good contact with the object or dip well in the liquid whose temperature is being measured.
2. The thermometer should not touch the wall or sides of the container.
3. The thermometer should be held vertically.
4. The reading of mercury level in the capillary tube should be taken by taking the reading of the upper meniscus of mercury, keeping the eye on level with the meniscus.

Types of thermometer according to the choice of thermometric liquid

There are two types of thermometers according to the thermometric liquid used. They are:

1. Mercury thermometer
2. Alcohol thermometer

#### Mercury thermometer

A mercury thermometer is used to measure the temperature of extreme hot places. Mercury thermometer can measure the temperature of -390 Celsius to 3570 Celsius.

Advantages of use of Mercury in thermometer

1. It is shiny liquid.
2. It does not stick to the wall of the glass tube.
3. It expands uniformly.
4. It can be used for a wide range of temperature.

#### Alcohol thermometer:

An alcohol thermometer is used to measure the temperature of extreme cold places. Alcohol thermometer can measure the temperature of - 1120 Celsius to 78.30 Celsius.

Advantages of use of Alcohol in thermometer

1. It is transparent; therefore, it has to be coloured before it is used.
2. It has low specific heat capacity.
3. It expands more than mercury (about six times) for the same rise in temperature.
4. Alcohol freezes at - 1120 Celsius and boils at 78.30 Celsius.

Calibration of thermometer:

Marking of temperature scale on a thermometer is called calibration. Calibration is done in two ways.

1. By marking two fixed points i.e. lower fixed point and upper fixed point.
2. By dividing the interval between the lower and the upper fixed points either in 100 equal parts and labelling each part as one degree Celsius in Celsius scale or in 180 equal parts and labelling each part as one degree Fahrenheit in Fahrenheit scale.

To mark the fixed points

1. Lower fixed point:The lower fixed point is the temperature of melting point of pure ice at standard atmospheric pressure of 760 mm of mercury.
2. Upper fixed point:The upper fixed point is the temperature of the steam produced from the pure boiling water at the standard atmospheric pressure of 760 mm of mercury.

Importance of thermometer:

1. It is used to measure the temperature of the body.
2. It is used to measure the temperature of a room.
3. It is used to know the temperature of the water of an aquarium.
4. It is used to know the temperature of the human body and environment.

##### Things to remember
• A device, which measures the temperature of a body, is called thermometer.
• Three temperature scales are used on thermometers: the Celsius, the Fahrenheit and the Kelvin scale.
• There are two types of thermometers  according to the thermometric liquid used: Mercury and Alcohol Thermometer.
• Mercury thermometer can measure the temperature between -390 Celsius to 3570 Celsius.
• Alcohol thermometer can measure the temperature between - 1120 Celsius to 78.30 Celsius.
• It includes every relationship which established among the people.
• There can be more than one community in a society. Community smaller than society.
• It is a network of social relationships which cannot see or touched.
• common interests and common objectives are not necessary for society.
##### Temperature Scales

Alcohol thermometer is preferred for a very cold region because its freezing point of alcohol is - 117°C.  So, it can measure the temperature of the very cold region.

Temperature of boiling water can’t be measured using an alcohol thermometer because the temperature of boiling water is at 100°C and that of alcohol is 78°C . So, alcohol boils before water and can’t be measured the boiling point of water.

Constriction presents in a clinical thermometer let the level of mercury increase when the temperature increases but doesn’t let it fall when temperature decreases and it helps to take the correct reading.

A clinical thermometer is jerked before using it to bring the level of mercury in the bulb.

Steel pointers are used in maximum and minimum thermometer to locate the temperature of the region.

Importance of thermometer are as follow:

• A clinical thermometer is used in the clinic or home to measure the temperature of human body.
• A laboratory thermometer is used in the laboratory to measure the temperature of a different substance.
• Maximum and minimum thermometer is used to measure the maximum and minimum temperature of the region.

Thermometer is based on the principle of, ' when a body is heated it expands and when a body is cooled it contracts ’.

I use mercury thermometer to measure the temperature above 80°C because the boiling point of alcohol is 78°C . So, it can’t measure the temperature above 78°C.

Maximum and minimum thermometer is used when maximum and minimum temperature of a region is required.

The advantages of using alcohol instead of mercury in a thermometer are as follows:

• The freezing point of alcohol is 117°C. Thus, it remains liquid to a very low temperature. Due to this reason alcohol thermometer is used for measuring the temperature in very cold regions.
• Its expansion rate is about seven times more than that of mercury.

The advantage of using alcohol instead of mercury in a thermometer is as follows:

• It is colorless. Hence, it should be colored before use in order to see it easily.
• It is a bad conductor of heat.
• It wets the wall of the capillary glass tube of the thermometer.
• Its expansion rate is not uniform.
• Its boiling point is 78°C. So, it can’t be used for measuring the temperature above 78°C. Due to this reason the alcohol thermometer is not used to measure temperature in very warm places.
• It has low density.

I prefer to use laboratory thermometer to measure the temperature of boiling water.

I prefer to use clinical thermometer to measure the temperature of human body.

They are as follows:
• It is a good conductor of heat.
• It has uniform rate of expansion and contraction.
• Mercury does not wet glass. So, the rise and the fall of the mercury in the tube is clean and smooth.
• It has very high density, so a small bulb of a thermometer can contain much mercury.
• Mercury remains liquid state over a quite wide range of temperature because it freezes at 39°C and boils at 357°C.

Solution,

\begin{align*} \frac {C - 0}{100} &= \frac {F-32}{180} \\ \text {Here,}\: F =98.6\: \text {Fahrenheit} \\\frac {(C- 0)} {100} &= \frac {(98.6- 32)}{180} \\\text {or,}\:\frac {C}{100} &= \frac {66.6}{180} \\ \text {or,}\: C \times 180 &= 66.6 \times 100\: ( \text {cross multiplication}) \\ \text {or,}\: C &= \frac {666}{18} = 37^o \:\text {Celsius} \\\end{align*}

Solution,

\begin{align*} \frac {C - 0}{100} &= \frac {F-32}{180} \\ \text {Here,}\: F = 0 \: \text {Fahrenheit} \\\frac {(C- 0)} {100} &= \frac {(0- 32)}{180} \\\text {or,}\:\frac {C}{100} &= \frac {-32}{180} \\ \text {or,}\: C \times 180 &= -32\times 100\: ( \text {cross multiplication}) \\ \text {or,}\: C &= \frac {-320}{18} = -17.7^o \:\text {Celsius} \\\end{align*}

Solution,

\begin{align*} \frac {F-32}{180} &= \frac {K-273}{180} \\ \text {Here,}\: F = 200\: \text {Fahrenheit} \\\frac {(200 - 32)} {180} &= \frac {(k- 273)}{100} \\\text {or,}\: K - 273 &= (200 - 32) \times \frac {100}{180} \\ \text {or,} \: k -273 &= (200 - 32) \times \frac {100}{180} \\ k &= 93.3 + 272 = 366.3 \\ \text {Thus,}\: 200\: \text {Farhrenheit} &= 366.6 \: \text {kelvin}\\\end{align*}

\begin{align*} \frac {C - 0}{100} &= \frac {k-273}{100} \\ \text {Here,}\: k = 100\: \text {kelvin} \\\frac {(C- 0)} {100} &= \frac {(100 -273)}{100} \\\text {or,}\: C - 0 &= \frac {-173\times 100}{100} \\ \text {or,}\: C &= -173 \\ \therefore 100\: \text {kelvin} = -173^o \:\text {Celsius} \\\end{align*}

Solution,

\begin{align*} \frac {C - 0}{100} &= \frac {F-32}{180} \\ \text {Here,}\: C = -40\: \text {Fahrenheit} \\\frac {(-40- 0)} {100} &= \frac {(F- 32)}{180} \\\text {or,}\:\frac {-40}{100} &= \frac {F -32}{180} \\ \text {or,}\: -40 \times 180 &= (F-32)\times 100\: ( \text {cross multiplication}) \\ \text {or,}\: F - 32 &= -4 \times 18 \\ \text {or,}\: F &= -40 \\ \therefore \text {The celcius and farenheit degree coincides when the value is}\: -40 \end{align*}

Solution

\begin{align*} \frac {C - 0}{100} &= \frac {K-273}{100} \\ \text {Here,}\: C =200\: \text {Fahrenheit} \\\frac {(200 - 0)} {100} &= \frac {(K-273)}{100} \\\text {or,}\: K - 273 &= 200 \\ \text {or,} \: K &= 200 + 273 \\ \text {or,}\: K &= 473\: \text {kelvin} \\ \therefore 200^o \text {is equal to}\: 473\: \text {kelvin}\\\end{align*}