4C45.10 Phase Change in Iron


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.



  • [1] Iron wire
  • [2] Post and socket
  • [1] Small weight on a hook
  • [1] Variac with spare fuses
  • [1] Slot-head screwdriver
  • [1] Wire cutter
  • [1] Extension cord
  • [1] Camera

Classroom Assembly

  1. Cut off about 8 turns of iron wire.
  2. Set up the posts and sockets, securing the wire to the posts.
  3. Make sure the variac is off.
  4. Plug the demo into the variac and the variac into mains power.

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.
  4. Leave the wire at high temperature for a few seconds to allow the entire length of the wire to heat up.
  5. Turn the variac to 0 V. The wire will start contracting. When the wire passes through the BCC-FCC transition, the wire will suddenly drop.


Additional Resources


  • PIRA 4C45.10


  • Don't attempt this at home!

Last revised

  • 2020


  • We use Royal Brand Music Spring Wire, music wire gauge no. 12, 0.029 inches diameter.
  • The wire briefly drops as it cools because the phase transition is exothermic. One can do the experiment with a copper wire to avoid the recalescence of iron.

Related AV

Related demos


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.