4C45.10 Iron Wire


Thermal expansion, solid-solid phase transition


When a length of iron wire is heated, it shows a sudden reduction in length through the bcc-fcc phase transition. The iron wire is heated by means of a variac. The wire is suspended between two posts and the posts are connected across the variac. A small weight on a hook is placed in the middle of the wire to increase the visibility of the sudden length reduction. Note: this demonstration requires a video camera to be clearly visible in large lecture halls.



  • [1] Iron wire
  • [2] Posts and sockets
  • [1] Small weight on a hook
  • [1] Variac with spare fuses
  • [1] Screwdriver
  • [1] Extension cord

Important Notes

  • The transition is more visible on cooling where a sudden drop in the wire is observed. Leave the wire at a high temperature for a few seconds to allow the entire length of the wire to heat up.
  • DO NOT turn the variac on with the voltage turned up to prevent blowing a fuse.
  • DO NOT touch the wire - it is HOT both electrically and in temperature.


  1. Set the variac to 0 V.
  2. Turn on the variac.
  3. Slowly turn up the variac to about 70 V. The wire will heat and begin to expand (sag). When the wire passes through the bcc-fcc phase transition, it will suddenly contract and 'jump' up.


Additional Resources


  • PIRA 4C45.10


  • Don't attempt this at home!

Last revised

  • 2018


  • We use Royal Brand Music Spring Wire, Music wire gauge No 12, 0.029 inches diameter.
  • Eight turns off the roll is about right.

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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.