where K = 1 + α α ⋅ ⋅ Δ Δ T , α α is the coefficient of thermal expansion. Analyzing this phenomenon in an absolute coordinate system, the transformation of the solid body is a geometrical similarity. It is possible that, choosing the constrains conveniently, one point belonging to the space associated to the body will not move during the thermal variation: this point is called thermal center. In this case, the geometrical similarity becomes a homothety and the thermal center is the center of the homothety itself.