4A30.XX Cool Plastic Rulers


Thermal expansion and contraction


One ruler is cooled to liquid nitrogen temperature and another is left at room temperature. The two rulers are compared to show that the cooled ruler has contracted.



  • [2] 12" rulers
  • [1] Small (~2 L) dewar of liquid nitrogen

Safety Equipment

  • [1] Safety glasses
  • [2] Safety glove

Important Notes

  • This demonstration requires advance practice. Aligning the rulers in time is difficult. The cooled ruler heats up very quickly.
  • If the cooled ruler curls or bend, hold the ruler flat on the overhead.
  • You can expect a contraction of about 1 mm for every 10 cm of ruler cooled.
  • Liquid nitrogen can cause severe burns and blindness. Use with caution.


  1. Place both rulers on the overhead projector so they touch with the cm scales aligned side-by-side. Note that the rulings line up exactly.
  2. Put on your safety goggles and gloves.
  3. Place one ruler in the dewar of liquid nitrogen with the 0 cm end down. Note that the entire ruler will not be submerged.
  4. Wait until the ruler is fully cooled to liquid nitrogen temperature.
  5. Remove the cooled ruler from the liquid nitrogen.
  6. Place both rulers on the overhead projector and align them as before. Note that the rulings no longer line up. The cooled ruler is now shorter.

Additional Resources


  • PIRA 4A30.XX


  • Don't attempt this at home!

Last revised

  • 2020


  • Original construction: assembled from existing equipment.

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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.