Such bonds shall be issued in amounts of not less than one hundred dollars and not over one thousand dollars each, and may be negotiable in form, and shall become due at such time, not exceeding twenty years from their date, as may be provided by ordinance, and shall bear interest, payable annually or semiannually, at a rate not exceeding six percent per annum; the principal and interest thereof being payable at such place in the United States and in such lawful money of the United States as may be specified in the ordinance directing their issue and in the bonds, and shall be executed under the seal of the city or town and by such officers or officer and in such manner as may be directed by ordinance.