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.
-  Iron wire
-  Posts and sockets
-  Small weight on a hook
-  Variac with spare fuses
-  Screwdriver
-  Extension cord
- 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.
- Set the variac to 0 V.
- Turn on the variac.
- 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.
- PIRA 4C45.10
- Don't attempt this at home!
- We use Royal Brand Music Spring Wire, Music wire gauge No 12, 0.029 inches diameter.
- Eight turns off the roll is about right.
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.