Measuring scale should be made up of the metal whose linear expansivity is negligibly small?
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Answer:
Measuring or properly called weigh scales use a strain gage if electronic or a set of levers (aka balance) to translate the weight of an object to a readable ‘scale’ or output device. Either way, if materials with high thermal expansion coefficients such as aluminum were used, the level arm length would change with even small changes in temperature. The load cell is a metal bar as a cantilever with strain gages on it that are sensitive to temperature themselves so the electronics need to compensate for the temperature changes such as you might experience outdoors. What is not obvious and is likely the reason for this question is the detail that these levers that translate a motion against a spring (the cantilever of the load cell or an actual spring) multiply the displacement (as a lever does) from something too small to see to something big enough to see on the display. This magnification of strain displacement also magnifies thermal expansion displacement and the final reading will shift because it is a sum of these two sources of displacement. So, by selective, intelligent choice of materials, the thermal expansion effect can be designed out of the instrument. Glass and ceramics and some metals like Invar have near zero thermal expansion coefficients. few plastics have such low coefficients so making a cheap scale from plastic is not a good idea. Besides, the levers in a mechanical scale need to be stiff and not bend under load so they are usually made from metal