1. HOW DOES THE THERMOMETER WORK

2. What is a thermometer?
A thermometer is a device that measures temperature of things.
Temperature is a measure of how fast the molecules in a liquid are moving

3. The First Thermometers
One of the earliest thermometers was probably invented by Galileo.
Earlier thermometers consisted of an open tube, filled with water. The instrument also had a large bulb on the end.

4. Problems with earlier thermometers
There were basically three problems with early thermometers:
Since water was used, the thermometer could not measure temperatures above 212oF because it would boil. It could not measure temperatures below 32oF because it would freeze.

5. Problems with earlier thermometers
The bulb was very large so it took a long time for the thermometer to reach the same temperature as the object that was being measured.
Because the top of the tube was open, water could evaporate and pick up dust and debris.

6. HOW THE PROBLEMS WERE HANDLED
To correct these problems, modern thermometers:
A. Use a closed tube
B. Use alcohol or mercury which has a
different freezing and boiling point than
water
C. Use a relatively small bulb which heats up
very quickly

7. Main types of thermometers
There are basically two main types of thermometers.
A Bulb Thermometer
A Bimetallic Strip Thermometer

8. Bulb Thermometers
Bulb thermometers rely on the principle that a liquid changes its volume relative to its temperature.
Liquids take up less space when they are cold and more space when they are warm.

9. Bulb Thermometers
A simple bulb thermometer is made up of an enclosed glass tube with a slightly enlarge tip. The tip is called the “bulb”. The tube is filled with a liquid, usually alcohol or mercury.
As temperature rises, the liquid in the tube rises and it falls as temperatures decrease.

10. Bulb Thermometers
Bulb thermometers are mainly used to MEASURE temperature.

11. Bimetallic Strip Thermometers
The bimetallic strip thermometer relies on the principle that different metals expand at different rates as they warm up.
These are the types of
thermometers used in ovens,
heaters, and airconditioners.
Bimetallic strips thermometers are used to control temperature

12. Bimetallic Strip Thermometers
A bimetallic strip thermometer is made of two different metals which expand and contract at different rates.
For example, when an oven is turned on, it will continue to heat until the metal in the thermometer expands enough to hit the contact and stop the temperature from continuing to rise. The metal expands in correlation to the temperature you set the control knob on.

13. Bimetallic Strip Thermometer
In a heating or cooling unit, the thermometer has two different metals. One metal will expand faster than the other which causes it to curl up and hit the contact to turn the heat on. When the desired temperature is reached, the metal stops expanding and turns the heat off. When the temperature starts to cool, the metal will contract, which will cause the unit to cut back on.

14. Three Main Temperature Scales
NOTE: The only scale that has absolute zero is the Kelvin scale. Absolte zero is the temperature at which hypothetically all molecular motion ceases, equal
to 0 degrees K.
Scale
Freezing Temperature
Boiling Temperature
Normal Body Temperature
Fahrenheit 32
212
98.6
Celsius
100 37
Kelvin
273
373
310