Build your own thermometer

Following the instructions below, your students can build their own thermometer. They can then compare its performance against the Sky Watchers thermometer.


Supplies needed to build your own thermometer
  • Glass jar (the smaller and narrower, the better)
  • A small quantity of cooking oil
  • Lid, stopper or cork for the jar
  • A sealant such as petroleum jelly, candle wax or modeling clay
  • Several drops of food colouring
  • Clear narrow drinking straw at least 15 centimetres long
  • Eye dropper
  • Water
  • An index or recipe card about 8cm by 13cm
  • Thermometer for reference


  1. Fill the glass jar with water and add a few drops of food colouring to make the water visible.
  2. Make a hole in the lid, stopper or cork, just large enough to slip the straw through.
  3. Place the stopper in the jar and insert the straw through the hole.
  4. Add more water, but this time through the straw and until the water is about one quarter of the way up the straw.
  5. Seal the straw into the stopper and the stopper onto the jar using either the petroleum jelly, modeling clay or candle wax.
  6. Finally put a drop of the cooking oil into the straw so that the oil sits on top of the water. The oil prevents the water from evaporating.
  7. Attach the index card to the straw. Allow the thermometer to settle for 2 or 3 hours.
  8. Now use your reference thermometer to calibrate your home-made thermometer. To do this, note the level of water in the straw and mark a line on the card. Beside the line, record the temperature shown on your reference thermometer. Repeat this process over the next several days.

A final note

The final product of the activity, demonstrating how the fluid in the straw moves up or down depending on the temperature

The width of the straw and the amount of liquid in the jar will affect how quickly and accurately your thermometer will respond. With a narrow straw, a smaller volume of water is required to raise the level in the straw noticeably.

Points to discuss

This thermometer is based on the principle that water, in fact most liquids, expand when heated and contract when cooled. Ask your students to predict where they think the hottest and coldest parts of the room are located - then let them check out their predictions over the next 2 days using their thermometer. Remind them that this thermometer takes a long time to respond because the entire jar of water must adjustbefore it will register the new temperature.

Ask your students if there are any drawbacks to using their home-made thermometer, and see if they can identify at least 3:

  • it's bulky
  • it's delicate, because it is made of glass
  • the water would freeze in winter or at temperatures below 0°C
  • it's slow to adjust to changes in temperature.

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