Experiment: Estimating the boiling temperature of nitrogen
A steel tray is used, with a transparent hinged lid, on which a scale is attached.
A steel-aluminium bi-material strip is used, with the two constituent strips having the same thickness and the strips are straight at room temperature. The steel-aluminium strip is fixed securely in the tray, using bolts and wing nuts, locating a spacer block between the side of the tray and the strip, as below.
The material with the smaller value of α (steel in this case) is arranged to be closest to the wall of the tray, so that strip will curve away from the nearest wall when cooled.
Liquid nitrogen is carefully poured into the tray, so that the strip is partially immersed, avoiding pouring directly onto the strip where possible. It is IMPORTANT that safety glasses and thick gloves are worn throughout this procedure. (Splashing of small amounts of liquid nitrogen onto clothes or skin is not particularly dangerous, but touching very cold metal with unprotected hands can cause severe injury.) The bi-material adopts a uniform curvature, arising from the difference in the free (unconstrained) contractions of each material in the strip. The deflection δ is recorded using the scale on the lid of the tray.