Definition of thermometer
“Thermometer is the device which is used to measure the temperature of a body.”Some substances have property that changes with temperature.Substances that show a change with temperature can be used as a thermometric material.For example,some substances expand on heating,some change their colors,some change their electric resistance ,etc.Nearly all the the substances expand on heating.Liquids also expand on heating and are suitable as thermometric materials.Common thermometers are generally made using some suitable liquid as thermometric material.
Which properties should present in thermometric liquid?
A thermometric liquid should have the following properties:
- It should be visible.
- It should have uniform thermal expansion.
- It should have a low freezing point.
- It should have a high boiling point.
- It should not wet glass.
- It should be a good conductor of heat.
- It should have a small specific heat capacity.
Which liquids are used in glass thermometer?
A liquid in glass thermometer has a bulb with a long capillary tube of uniform and fine bore.A suitable liquid is filled in the bulb.When the bulb contacts a hot object,the liquid in it expands and rises in the tube.The glass stem of a thermometer is thick and acts as a cylinder lens.This makes it easy to see the liquid level in the glass tube.Mostly Mercury and Alcohols are used in glass thermometers.
Why mercury used in thermometers instead of water ?
Mercury is used mostly in thermometers because of following properties:
- It is visible.
- It has low freezing point (-39 °C).
- It has very high boiling point (357 °C).
- It expands linearly.
- It gives accurate measurements.
- It is a good conductor of heat.
- Fast response time.
- It has wide range of temperatures.
Properties of Alcohols for thermometers
- It has low freezing point (-112 °C).
- It has boiling point of (78 °C).
- It is used to measure high temperature.
- It is not good conductor of heat.
- It has bright colour.
- It expands more than mercury.
Categories of thermometers
There are three categories of thermometers which are given these:
- Clinical thermometer:
It is used to measure human body temperature in the range of 35 °C to 42 °C.
- Lab thermometer:
It is used to measure the room temperature of hot solids and liquids in experiments.It measures temperature in the range of 5 °C to 110 °C and on more higher temperatures.
- Digital thermometers:
These are advanced thermometers used to measure temperature of a body of high level of accuracy.
Types of thermometers with names and pictures
An ideal thermometer shall have infinite temperature range.Since no thermometer is ideal,therefore we have large number of thermometers.
It has linear expansion property and has temperature range of -35°C to +500 °C(with compressed nitrogen).It works on the principle:
ΔL α ΔT
Where ΔT= Δl/lοα and α is the linear expansion coefficient.
2. Constant pressure gas thermometer
In this thermometer volume increases directly with increase in temperature.It measures temperature in the range from o k 5oo k.
ΔV α ΔT
And γ=1/273 for ideal gases.
3. Constant volume thermometer
In this thermometer pressure increases directly with increase of temperature.It measures temperature in the range o k to 5oo k.
ΔP α ΔT
where ΔT=Δp/P triple point γ
=273 Δp/P triple point
4.Platinum resistance thermometer
In this thermometer resistance of material in thermometer increases directly with increase of temperature.It measures temperature in the range of 5oo k to 2300 k.
ΔR α ΔT
where ΔT=ΔR/Rα , α is thermal coefficient of resistance.
5. Thermocouple thermometer
This thermometer work on the principle:
emf ∈ = αT +β T²
In this scale is nonlinear.Temperature is either matched with a standard curve supplied by the manufacturer or a digital display is provided.α and β depend upon the materials used to form thermocouple. where:
emf ∈=αT + βT²/2
It measures temperature in the range 500 k to 2300 k
This thermometer work on the principle of radiation.It measures temperature greater than 2000 k.It uses Stefan’s law:
Intensity E= σT4