First the tray and specimen are brought back to room temperature by immersing in a bucket of water. (Warming up is complete by when the strip has become approximately straight again.)
It is IMPORTANT not to attempt to swap over specimens while the tray is cold.
The experiment is then repeated for the aluminium-polycarbonate strip. The thermal expansivity of polycarbonate can then be estimated, again assuming that the expansivity of aluminium given in the properties table is correct, and using the value for ΔT obtained previously.
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