Wherever in this chapter the following words are used, they shall be held to have the meanings hereinafter given:
"Public work" : Any public building, public highway, bridge, or other public betterment, work or improvement of a permanent nature, constructed, reconstructed, repaired or improved wholly at the expense of the public.
"Domestic materials" : Articles, materials and supplies mined or produced in the United States, and manufactured articles, materials and supplies manufactured in the United States.