Measurement of temperature

Fundamental Principle of Temperature Measurement

Temperature cannot be measured directly.

Instead, it is measured by observing a physical property that:

• changes with temperature,

• changes in a regular and measurable way,

• can be calibrated against known temperature values.

Any property that satisfies these conditions can be used to construct a thermometer.

What Is a Thermometric Property?

A thermometric property is a physical quantity that:

• varies continuously with temperature,

• changes in a predictable manner,

• can be measured accurately.

Examples include:

• length or volume (thermal expansion),

• electrical resistance,

• electromotive force (e.m.f.).

Thermal Expansion as a Thermometric Property

Expansion of Liquids

Liquids expand when heated and contract when cooled.

In a liquid-in-glass thermometer:

• the liquid expands with temperature,

• the increase in volume causes the liquid level to rise in a narrow capillary tube,

• the height of the liquid column indicates temperature.

This works because:

• expansion is gradual,

• expansion is repeatable,

• expansion is measurable.

[Insert diagram of a liquid-in-glass thermometer showing expansion of liquid with temperature]

Expansion of Solids (Bimetallic Strip)

Different solids expand by different amounts when heated.

In a bimetallic strip:

• two different metals are bonded together,

• one metal expands more than the other,

• the strip bends when temperature changes.

This bending:

• can be used to move a pointer,

• or open/close an electrical contact (as in thermostats).

[Insert diagram of a bimetallic strip bending due to heating]

Electrical Effects as Thermometric Properties (e.m.f.)

Thermocouple and e.m.f.

Some temperature sensors rely on electrical effects rather than expansion.

When two different metals are joined to form a thermocouple:

• a small electromotive force (e.m.f.) is produced,

• the value of the e.m.f. depends on the temperature difference between the junctions.

As temperature changes:

• the e.m.f. changes in a predictable way,

• this change can be measured using a sensitive voltmeter,

• the measured e.m.f. is used to determine temperature.

[Insert diagram of a thermocouple showing two metal junctions connected to a voltmeter]

Advantages of Using e.m.f.

• responds quickly to temperature changes,

• suitable for very high temperatures,

• can measure temperatures where liquid thermometers cannot be used.

Why These Properties Are Suitable for Measuring Temperature

A physical property used to measure temperature must:

• vary continuously with temperature,

• vary in a known and predictable manner,

• be easy to measure accurately,

• be reproducible.

Thermal expansion and e.m.f. satisfy all these conditions.

Summary Table: Thermometric Properties

Key Exam-Ready Statements

• Temperature is measured using a physical property that changes with temperature.

• Thermal expansion and electrical e.m.f. are common thermometric properties.

• These properties change predictably and measurably.

• Calibration allows temperature values to be assigned.

Questions

Question 1

What is meant by a thermometric property?

Question 2

Describe how thermal expansion is used to measure temperature.

Question 3

Explain how e.m.f. can be used in the measurement of temperature.

Solutions

Solution 1

A thermometric property is a physical property that changes in a measurable and predictable way with temperature.

Solution 2

In a liquid-in-glass thermometer, the liquid expands when heated and contracts when cooled.

The change in volume causes the liquid level to rise or fall in a capillary tube, indicating temperature.

Solution 3

In a thermocouple, two different metals joined together produce an e.m.f. that depends on temperature.

As temperature changes, the e.m.f. changes predictably and can be measured using a voltmeter.

This measured e.m.f. is used to determine the temperature.

Examiner-Level Guidance

• Always state that temperature is measured indirectly.

• Do not confuse heat with temperature.

• Mention predictable variation for full marks.

• Diagrams greatly improve clarity in explanations.

Why Fixed Points Are Needed

Temperature is measured using thermometers, which rely on a thermometric property (such as expansion or e.m.f.).

However, different thermometers:

• may use different materials,

• may respond differently to temperature changes.

Without reference standards, measurements would not be consistent or comparable.

Fixed points are therefore required to:

• provide reference temperatures,

• allow thermometers to be calibrated,

• ensure uniformity and accuracy across all measurements.

What Is a Fixed Point?

A fixed point is a reproducible temperature that occurs under specific physical conditions and can be obtained reliably anywhere.

A good fixed point must:

• occur at a well-defined physical change,

• be easy to reproduce,

• remain constant under the same conditions.

Common Fixed Points on the Celsius Scale

1. Lower Fixed Point (Ice Point)

The lower fixed point is:

• the temperature of pure melting ice at normal atmospheric pressure,

• defined as 0 °C.

At this point:

• ice and water coexist in equilibrium,

• temperature remains constant during melting.

[Insert diagram showing ice and water in equilibrium at 0 °C with thermometer]

2. Upper Fixed Point (Steam Point)

The upper fixed point is:

• the temperature of pure boiling water (steam) at normal atmospheric pressure,

• defined as 100 °C.

At this point:

• water and steam coexist,

• temperature remains constant during boiling.

[Insert diagram showing boiling water producing steam at 100 °C with thermometer]

Use of Fixed Points to Define a Temperature Scale

On the Celsius scale:

• the interval between 0 °C and 100 °C is divided into 100 equal parts,

• each part represents 1 degree Celsius.

This creates a uniform and reproducible temperature scale.

Fixed Points on the Kelvin Scale (Concept Link)

While the Celsius scale uses two fixed points, the Kelvin scale is based on:

• absolute zero (0 K) as the fundamental fixed point.

However, practical calibration still relies on reproducible reference temperatures, similar to those used on the Celsius scale.

Why Fixed Points Ensure Reliable Measurement

Fixed points allow:

• different thermometers to be calibrated in the same way,

• temperature readings to be comparable worldwide,

• scientific experiments to be reproducible.

Without fixed points:

• thermometer readings would be arbitrary,

• temperature measurements would lack scientific meaning.

Summary Table: Fixed Points

Key Exam-Ready Statements

• Fixed points are reference temperatures used to define a temperature scale.

• They are needed for calibration and accuracy.

• On the Celsius scale:
  • ice point = 0 °C,
  • steam point = 100 °C.

• Fixed points ensure uniform temperature measurement.

Questions

Question 1

What is meant by a fixed point of a temperature scale?

Question 2

State the two fixed points used to define the Celsius temperature scale.

Question 3

Explain why fixed points are needed in the measurement of temperature.

Solutions

Solution 1

A fixed point is a constant, reproducible temperature used as a reference in defining and calibrating a temperature scale.

Solution 2

The two fixed points are:

• the melting point of pure ice (0 °C),

• the boiling point of pure water at normal atmospheric pressure (100 °C).

Solution 3

Fixed points are needed to provide reference temperatures for calibrating thermometers.

They ensure that different thermometers give consistent and comparable readings.

Without fixed points, temperature measurements would not be accurate or reliable.

Examiner-Level Guidance

• Always mention calibration when explaining the need for fixed points.

• State conditions (pure substance, normal atmospheric pressure).

• Do not confuse fixed points with thermometric properties.

• Diagrams of ice and steam points improve clarity.

Why Thermometer Characteristics Matter

Not all thermometers are suitable for all situations.

A thermometer must be chosen based on:

• how small a temperature change it can detect,

• the temperatures it can measure safely,

• how regularly it responds to temperature changes.

These qualities are described by sensitivity, range, and linearity.

1. Sensitivity

Meaning of Sensitivity

Sensitivity refers to how much a thermometer’s reading changes for a small change in temperature.

A thermometer is highly sensitive if:

• a small temperature change produces a large and easily detectable change in the thermometric property.

For example:

• a thermometer with a narrow capillary tube shows a large rise in liquid level for a small temperature increase.

Importance of Sensitivity

High sensitivity is important when:

• measuring small temperature changes,

• detecting slight variations accurately.

However:

• very high sensitivity may reduce the range of the thermometer.

[Insert diagram comparing a sensitive thermometer with a wide liquid movement to a less sensitive one]

2. Range

Meaning of Range

The range of a thermometer is the interval of temperatures it can measure reliably.

For example:

• a clinical thermometer has a small range (around body temperature),

• a thermocouple has a very wide range, including extremely high temperatures.

Importance of Range

A thermometer must have a range suitable for its purpose:

• low range → medical use,

• wide range → industrial or laboratory use.

Using a thermometer beyond its range:

• can damage the instrument,

• gives inaccurate readings.

[Insert diagram showing thermometer scales with narrow range vs wide range]

3. Linearity

Meaning of Linearity

A thermometer is linear if:

• equal changes in temperature produce equal changes in the thermometric property.

In a linear thermometer:

• the scale markings are evenly spaced,

• readings are easier and more accurate.

Importance of Linearity

Good linearity:

• simplifies calibration,

• reduces reading errors,

• allows accurate interpolation between scale marks.

If a thermometer is not linear:

• its scale becomes uneven,

• accuracy decreases unless corrections are applied.

[Insert graph of thermometric property vs temperature showing linear and non-linear relationships]

Comparison Summary (Exam-Critical)

Key Exam-Ready Statements

• Sensitivity shows how responsive a thermometer is.

• Range determines the temperatures a thermometer can measure.

• Linearity ensures equal temperature changes give equal scale changes.

• No thermometer can maximise all three perfectly at the same time.

Questions]

Question 1

What is meant by the sensitivity of a thermometer?

Question 2

Explain why a thermometer with a very high sensitivity may have a limited range.

Question 3

Describe what is meant by linearity in a thermometer and state one advantage of a linear scale.

Solutions

Solution 1

The sensitivity of a thermometer is the size of change in its reading for a small change in temperature.

Solution 2

A highly sensitive thermometer shows a large change in reading for a small temperature change.

This means the thermometric property reaches its maximum value quickly, limiting the range of temperatures that can be measured.

Solution 3

Linearity means equal changes in temperature produce equal changes in the thermometric property.

An advantage of a linear scale is that temperature readings are easier to read and more accurate.

Examiner-Level Guidance

• Always define the term first, then explain.

• Do not confuse sensitivity with accuracy.

• Range answers must mention upper and lower limits.

• Linearity explanations are strongest when linked to scale spacing.

Principle of Liquid-in-Glass Thermometers

Liquid-in-glass thermometers operate on thermal expansion of a liquid.

When temperature changes:

• the liquid expands on heating and contracts on cooling,

• the change in volume is converted into a change in length in a narrow tube,

• the liquid level indicates temperature on a calibrated scale.

Common liquids used:

• Mercury (uniform expansion, clear meniscus, suitable for higher temperatures),

• Alcohol (larger expansion, visible dye, suitable for lower temperatures).

Structure of a Liquid-in-Glass Thermometer (General)

A typical liquid-in-glass thermometer consists of:

1. Bulb
   • A thin glass bulb at the bottom.
   • Contains the thermometric liquid.
   • Large volume ensures noticeable expansion.

2. Capillary Tube
   • Very narrow glass tube above the bulb.
   • Converts small volume changes into large length changes (high sensitivity).

3. Thermometric Liquid
   • Mercury or alcohol.
   • Expands uniformly with temperature.

4. Glass Stem with Scale
   • Graduated temperature scale marked alongside the capillary.
   • Allows direct reading of temperature.

[Insert labelled diagram of a liquid-in-glass thermometer showing bulb, capillary tube, liquid column, and scale]

Action (Working) of a Liquid-in-Glass Thermometer

• When the thermometer is placed in contact with a warmer object:
  • heat flows into the bulb,
  • the liquid expands,
  • the liquid level rises in the capillary tube.

• When placed in contact with a cooler object:
  • the liquid contracts,
  • the liquid level falls.

The height of the liquid column corresponds to a temperature value on the scale.

Laboratory Thermometer

Structure

A laboratory thermometer:

• has a uniform capillary tube,

• does not have a constriction,

• usually uses mercury or alcohol,

• has a wide temperature range (e.g. –10 °C to 110 °C).

Action

• The liquid level rises and falls freely with temperature changes.

• The thermometer must remain in contact with the substance while reading.

Uses

• Measuring temperatures of liquids and gases in experiments.

• General laboratory work.

[Insert diagram of a laboratory thermometer with wide range and no constriction]

Clinical Thermometer

Structure

A clinical thermometer is specially designed to measure human body temperature.

Key features:

• Narrow capillary tube for high sensitivity,

• Mercury as the thermometric liquid,

• Constriction (kink) just above the bulb,

• Small temperature range (typically 35 °C to 42 °C).

The constriction is the most important distinguishing feature.

[Insert labelled diagram of a clinical thermometer showing bulb, constriction, capillary tube, and scale]

Action of the Clinical Thermometer

• When placed in the body:
  • mercury expands and rises past the constriction.

• When removed:
  • the constriction prevents mercury from flowing back immediately.

• This allows the temperature to be read after removal.

To reset the thermometer:

• it must be shaken, forcing mercury back into the bulb.

Comparison: Laboratory vs Clinical Thermometer

Key Exam-Ready Statements

• Liquid-in-glass thermometers work on thermal expansion.

• The capillary tube increases sensitivity.

• A clinical thermometer has a constriction to retain the reading.

• A laboratory thermometer has no constriction and a wider range.

Questions

Question 1

Describe the structure of a liquid-in-glass thermometer.

Question 2

Explain how a liquid-in-glass thermometer measures temperature.

Question 3

State two structural differences between a laboratory thermometer and a clinical thermometer.

Question 4

Explain the function of the constriction in a clinical thermometer.

Solutions

Solution 1

A liquid-in-glass thermometer consists of a glass bulb containing a liquid, a narrow capillary tube, and a graduated scale along the glass stem. The bulb holds the liquid, and the capillary tube allows small volume changes to be observed as length changes.

Solution 2

When temperature increases, the liquid in the bulb expands and rises up the capillary tube.

When temperature decreases, the liquid contracts and falls.

The liquid level indicates temperature on the calibrated scale.

Solution 3

Two differences are:

• a clinical thermometer has a constriction, while a laboratory thermometer does not,

• a clinical thermometer has a narrow temperature range, while a laboratory thermometer has a wide range.

Solution 4

The constriction prevents the mercury from flowing back into the bulb when the thermometer is removed.

This allows the body temperature reading to be taken after removal from the patient.

Examiner-Level Guidance

• Always mention thermal expansion as the principle.

• Clinical thermometer answers must include constriction and narrow range.

• Do not confuse thermometer structure with calibration.

• Diagrams with labels significantly improve marks.

What Is a Thermocouple?

A thermocouple is a temperature-measuring device that uses an electrical effect rather than thermal expansion.

It measures temperature by producing an electromotive force (e.m.f.) when two different metals are joined and exposed to different temperatures.

Thermocouples are especially useful for:

• very high temperatures,

• rapid temperature changes,

• situations where liquid thermometers cannot be used.

Structure of a Thermocouple

A simple thermocouple consists of:

1. Two Dissimilar Metal Wires
   • Made from two different metals (for example, copper and iron).
   • The metals must be different to produce an e.m.f.

2. Hot Junction
   • The point where the two metals are joined together.
   • This junction is placed at the temperature to be measured.

3. Cold (Reference) Junction
   • The other ends of the metal wires.
   • Usually kept at a known or constant temperature.

4. Sensitive Voltmeter (or Galvanometer)
   • Connected in the circuit to measure the small e.m.f. produced.

[Insert labelled diagram of a thermocouple showing two dissimilar metals, hot junction, cold junction, and voltmeter]

Action (Working Principle) of a Thermocouple

Production of e.m.f.

When the hot junction and cold junction are at different temperatures:

• charge carriers in the metals gain different energies,

• an e.m.f. is produced in the circuit,

• this e.m.f. depends on the temperature difference between the two junctions.

Measuring Temperature

• As the temperature of the hot junction increases:
  • the e.m.f. produced increases,
  • the voltmeter reading increases.

• As the temperature of the hot junction decreases:
  • the e.m.f. decreases,
  • the voltmeter reading decreases.

By calibrating the voltmeter scale:

• the e.m.f. reading can be converted directly into a temperature value.

[Insert diagram showing increased e.m.f. when hot junction temperature increases]

Key Features of Thermocouples

• Very fast response time.

• Can measure very high temperatures.

• Small and durable.

• Suitable for industrial and laboratory use.

However:

• the e.m.f. produced is very small,

• a sensitive measuring device is required.

Comparison with Liquid-in-Glass Thermometers (Concept Link)

Key Exam-Ready Statements

• A thermocouple uses two different metals.

• Temperature measurement is based on e.m.f. produced by temperature difference.

• The e.m.f. depends on the difference between hot and cold junction temperatures.

• Thermocouples are suitable for high and rapidly changing temperatures.

Questions

Question 1

Describe the structure of a thermocouple.

Question 2

Explain how a thermocouple is used to measure temperature.

Question 3

State two advantages of using a thermocouple instead of a liquid-in-glass thermometer.

Solutions

Solution 1

A thermocouple consists of two different metal wires joined at one end to form a hot junction, with the other ends connected to a sensitive voltmeter to measure the e.m.f. produced.

Solution 2

When the hot and cold junctions are at different temperatures, an e.m.f. is produced in the circuit.

The size of the e.m.f. depends on the temperature difference.

By measuring this e.m.f. with a voltmeter and using calibration, the temperature of the hot junction can be determined.

Solution 3

Two advantages are:

• it can measure very high temperatures,

• it responds quickly to temperature changes.

Examiner-Level Guidance

• Always mention two different metals.

• Use the term hot junction correctly.

• Do not confuse e.m.f. with electric current.

• Emphasise temperature difference, not absolute temperature alone.

Limitations of Liquid-in-Glass Thermometers

Liquid-in-glass thermometers are not suitable for all situations because:

• liquids may boil or vaporise at high temperatures,

• glass may soften or break,

• the response time is slow due to thermal inertia,

• they cannot track rapid temperature fluctuations accurately.

These limitations create the need for an alternative temperature-measuring device.

Why Thermocouples Are Suitable for High Temperatures

Thermocouples are ideal for high-temperature measurement because:

• they are made of metals, which can withstand very high temperatures,

• no liquid is involved, so there is no boiling or expansion limit,

• the sensing junction is small and robust.

As a result, thermocouples can measure temperatures in:

• furnaces,

• engines,

• molten metals,

• industrial heating systems.

[Insert diagram showing a thermocouple hot junction placed inside a furnace or flame]

Why Thermocouples Respond Rapidly

Thermocouples are also ideal for rapidly changing temperatures because:

• the hot junction has a very small mass,

• it absorbs and loses heat quickly,

• the e.m.f. changes almost instantly with temperature.

This means:

• sudden temperature rises or falls are detected immediately,

• readings closely follow real-time temperature changes.

Liquid-in-glass thermometers respond slowly because the liquid and glass must warm up or cool down first.

[Insert diagram comparing slow response of liquid thermometer and fast response of thermocouple]

Practical Situations Requiring Thermocouples

Thermocouples are used where:

1. Temperatures are very high
   • inside furnaces,
   • combustion chambers,
   • kilns.

2. Temperatures change rapidly
   • engine exhausts,
   • chemical reactions,
   • electrical components under varying loads.

In these situations:

• accuracy,

• speed,

• durability
  are more important than high sensitivity to small temperature changes.

Key Physical Reasons (Exam-Critical)

Thermocouples are suitable because:

• metals withstand extreme heat,

• the e.m.f. responds directly and quickly to temperature change,

• no fragile glass or liquid is involved,

• electrical measurement allows instant detection.

Key Exam-Ready Statements

• Thermocouples are used for very high temperatures.

• They respond quickly to temperature changes.

• The hot junction has low thermal inertia.

• Liquid-in-glass thermometers are unsuitable in such conditions.

Questions

Question 1

State one reason why a thermocouple is suitable for measuring very high temperatures.

Question 2

Explain why a thermocouple responds more quickly to temperature changes than a liquid-in-glass thermometer.

Question 3

Give two situations where a thermocouple would be preferred to a liquid-in-glass thermometer and explain one of them.

Solutions

Solution 1

A thermocouple is suitable because it is made of metals that can withstand very high temperatures.

Solution 2

A thermocouple responds quickly because its hot junction has a very small mass and low thermal inertia.

This allows it to gain or lose heat rapidly, causing the e.m.f. to change almost immediately with temperature.

Solution 3

Thermocouples are used in furnaces and engine exhausts.

In a furnace, temperatures are extremely high and change rapidly, so a thermocouple can withstand the heat and respond quickly, unlike a liquid-in-glass thermometer.

Examiner-Level Guidance

• Always link fast response → small mass / low thermal inertia.

• High temperature answers must mention metal construction.

• Avoid vague phrases like “it works better”.

• Comparison with liquid-in-glass thermometers strengthens answers.