If the lengths of two bars of different solids are inversely proportional to their respective coefficients of linear expansions at the same initial temperature, the difference in length between them will be constant at all temperature. That means one way to construct a device with two points whose separation remains the same in spite of the temperature changes is to bolt together one end of two bars having different coefficients of linear expansions as shown below. What should be the lengths of a steel and a brass bar at 0 degrees Celsius so that all temperature difference in length is 0.30m.

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