Thermometers measure temperature using thermometric properties that change predictably with temperature such as the length of a mercury column, resistance of a wire, or pressure of a gas. Different thermometers use different thermometric substances and properties to develop temperature scales based on fixed points like the melting and boiling points of water. While individual thermometer scales may disagree, defining a standard thermometric substance, property, and scale removes these disagreements. Common thermometers include liquid-in-glass, gas, resistance, thermistor, and thermocouple thermometers.

1. Temperature
A2 Level Physics
Mukesh N. Tekwani
mukeshtekwani@hotmail.com

2. Thermodynamics
 A branch of physics in which thermal
effects are studied using macroscopic
quantities.
 Macroscopic quantities are:
• Pressure
• Temperature
• Volume
• Internal energy
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3. Kinetic Theory
 Kinetic theory is also concerned with
thermal effects.
 But here we assume the existence of
atoms and molecules.
 The laws of mechanics and statistics are
applied to a large number of these
molecules.
 Microscopic level
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4. Thermal Equilibrium
 If a body (X) at a higher temperature is in
contact with a body (Y) at a lower temperature,
then X will transfer heat energy to the lower
temp object (Y)
 This process will continue till the objects attain
the same temperature.
 The temp of both objects will become same.
 This thermal state in which both bodies are at
the same temp is called thermal equilibrium.
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5. Using Physical Properties to
Measure Temperature
 Many physical properties change with
temperature
• Change in volume (e.g. expansion of a liquid)
• Change in length of a mercury column
• Change in resistance of a wire
• Change in pressure of a gas at constant volume
 All these properties can be used in different
types of thermometers
 A thermometer is an instrument for
measurement of temperature.
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6. Imp Definitions
 The physical property on which a particular
thermometer is based is called the
thermometric property.
 Thermometric substance is the material used in
the thermometer, whose property varies with
temperature
 Thermometric property should vary linearly
with temp over a reasonable range of temp
 The range of linearity of the thermometric
property is called the thermometric range.
 E.g., mercury is a thermometric substance
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7. Temperature Scales
 Each type of thermometer can be used to
establish its own temp scale.
 How is this done?
• Every substance changes state at a fixed
temperature. E.g., solid-to-liquid OR liquid-
to-gas
• These fixed temperatures can be used to
define reference temperatures. These
reference temperatures are called fixed
points.
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8. Temperature Scales
 We take the value of the thermometric
property at the two fixed points and
divide the range into a number of EQUAL
steps or degrees.
 This way we set up an “empirical scale”
of thermometer.
 Empirical => ‘derived from experiment’
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9. Temperature Scales
 E.g., suppose we consider the fixed
points as the melting point of ice (00) and
boiling point of water (1000)
 Divide the range from 0 to 100 into 100
equal degrees. This gives the empirical
scale of temperature for that
thermometer.
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10. Disagreement between Thermometers
 The choice of thermometric susbstance
and thermometric property and the
assumed relation between the property
and temperature leads to an individual
temp scale.
 Measurements made with such a
thermometer may not agree with
measurements made by any other temp
scale
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11. Disagreement between Thermometers
 This disagreement is removed by the
following:
• Using a particular thermometric substance
• Using a particular thermometric property
• A particular relation between that property
and the temp scale. E.g., a linear relation
between length of a mercury column and
temp
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12. Determining an unknown Temp
 Consider the following graph which shows
the variation of a property P with temp.
Thermometric range
P100
= t2 – t1
Pt
P0
t1 0 t 100 t2
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Degrees C

13. Determining an unknown Temp
 From the st line graph, we get:
P100 - P0
= const
100 - 0
Pt - P0
= const
t-0
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14. Determining an unknown Temp
P100 - P0
= const
100 - 0
Pt - P0
= const
t-0
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15. Determining an unknown Temp
 Equating the LHS of these eqns we get
100 x (Pt – P0)
t =
(P100 – P0)
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16. Thermometric Properties
 Length varies linearly with temperature
• Give formula
• Used in liquid-in-glass thermometers
 Pressure varies linearly with temperature
• Give formula
• Used in constant volume gas thermometers
 Volume varies linearly with temperature
• Give formula
• Used in constant pressure gas thermometers
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17. Thermometric Properties
 Resistance varies linearly with temp
• Give formula
• Used platinum resistance thermometers
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18. Thermometers
 Factors to consider while choosing a thermometer
• Accuracy
• Sensitivity (distance between divisions on the scale)
• Range of temps it can measure
• Speed of response – measure rapidly varying
temperatures
• Sensitive part of the thermometer should be small so
that it does not absorb much heat from the object
• It shuld be easy to read – no complicated
calibrations / settings
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19. Platinum Wire Thermometers
 These are based on the variation of electrical resistance
due to temperature
 Resistance of a metal wire increases with increase in
temp.
Rt = R0 ( 1 + α t)
 Range of temp they can measure is from -260 C to
1700 C
 Temp sensor or material is a coil of fine platinum wire
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20. Platinum Wire Thermometers
 Advantages of platinum resistance thermometers:
• High accuracy
• L ow drift
• Wide operating range
 Disadvantages of Platinum Thermometers
• Not very sensitive to small changes in temp
• Slow response time
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21. Thermistor
 A thermistor is a type of resistor whose resisatnce
changes with temperature
 Therm (heat / temp) + Resistor = Thermistor
 Semiconductors are used in these devices
 The electrical resistance decreases very rapidly with
increasing temp.
 Thus, we say that thermistors have a negative temp
coeff. of resistance (NTC)
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22. Thermistors
 Advantages of thermistors:
• Sensitivity is high
• Their size is very small – hence can be used to measure temp
of small objects
 Disadvantages of Thermistors
• Their scale is non-linear
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23. Thermocouple
 This makes use of the thermoelectric effect
 Thermoelectric device - A thermoelectric device creates a
voltage when there is a different temperature on each side
 When the junctions of two different conductors, such as copper
and constantan wire are at different temperatures, an emf
(voltage) is developed.
 The relation between emf and temp diff is not linear.
 The emf generated is small – typically about 5 mV for a temp diff
of 100 C
 Advantages:
• Can be used to measure rapidly changing temps.
• Can be used to measure temp of small objects
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