Wednesday, 13 January 2016


Physics - Thermometers: liquid-in-glass

Key Terms:

Capillary tube - A glass tube with thick walls and a narrow interior hollow tube (bore). It has a bulb at one end. 

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To the left is a labelled diagram of a thermometer, you don't need to know all of the different parts of a thermometer, but I included it to help you understand.

How they work:

When the glass bulb is is heated, the liquid in the bulb starts to expand up the capillary tube. 

The liquid must have the following properties:

  • Be able to be seen easily
  • Be able to expand and contract quickly over a wide range of temperatures
  • Not stick to the inside of the capillary tube
Commonly Mercury and Alcohol (coloured) are used because Mercury boils at 357℃ and freezes at -39℃. Alcohol boils at 78℃ and freezes at -115℃, therefore Alcohol is more suited to low temperatures and Mercury is suitable for higher temperatures.

Clinical Thermometers

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Clinical thermometers are used by doctors and nurses to measure patient's body temperatures, they are mercury-in-glass thermometers. 

These are different from normal mercury-in-glass thermometers as their scale only extends for a few degrees either side of 37℃, this is because 37℃ is the normal body temperature. 

The capillary tube in these thermometers is very narrow, allowing for a very accurate reading to be taken, a.k.a the thermometer has a high sensitivity.

How they work:

The capillary tube has a constriction (check diagram above if you don't know what this is) just above bulb.

The thermometer is placed under the tongue and kept there for 1 minute, the mercury expands it heats up and forces past the constriction. Once the thermometer is removed from the mouth, the mercury in the bulb cools and contracts, breaking off from the mercury thread at the constriction, but the mercury that's beyond the constriction stays in the tube, showing the body temperature is at. 

By flicking your wrist, the mercury returns to the bulb again.

Mercury thermometers (for clinical use) are now being replaced by digital thermometers, because mercury is toxic.

Thermocouple Thermometers

Thermocouples are used to measure temperatures that change quickly or are very high.

Thermocouple thermometers have wires made of two different materials, e.g iron and copper that are joined together. If one junction is at a higher temperature than the other one, an electric current flows, producing a reading on a digital voltmeter, the voltage of the current is directly related to the temperature.

They are used in industry to measure a vast range of temperatures from -250℃ to 1500℃, they are also very good at measuring the temperatures on small objects or for rapidly changing temperatures.

Resistance Thermometers

Resistance thermometers are used to measure temperatures accurately from -200℃ to 1200℃.
They work by using the fact that electrical resistance builds up in a platinum wire when the temperature increases. 
They are best used for steady temperatures and are bulky.

Constant-Volume Gas Thermometers

Constant-Volume Gas thermometers use the change in pressure of a gas to measure temperature. 
It works over a wide rang of temperatures, and is bulky.


Thermistors work over a small range, ie. -5℃ to 70℃. It's resistance decreases with temperature.

Thermochromic Liquids

Thermochromic liquids change colour when heated or cooled, they have a limited range of temperatures (around room temperatures).

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