Measuring scale should be made up the metal whose linear expansivity is negligibly small. Why?

Because the lever would change with small changes.

Explanation:

When a material that has a rather high thermal expansion coefficients is used, the length of the level arm length changes with corresponding changes in its temperature. This, isn't a good phenomenon, and so as not to interfere, that's why there is a need for materials with lower thermal expansion coefficient.

no

sodium is solid at rt