Answer:
Step-by-step explanation:
a) When the iron rod is heated, it expands (gets larger). That's the key to this whole procedure. It doesn't expand much -- depending on what temperature the iron rod is heated up to.
When the rod looks like it has expanded as much as it can, the rod is tightened as much as it can be tightened before damage is done. When the rod cools, it contracts (gets smaller). That brings the walls closer together or it stops the walls from bulging.
Very interesting question. I've never seen this. Thanks for asking.