The president of the board, with the approval of the board, may designate one or more persons who shall have authority, when directed to do so by the president, to affix the signature of the president to any bond, warrant, certificate, contract or any other written instrument, which by law is required to be signed by the president of the board. When the signature of the president of the board is so affixed to a written instrument, it shall be as binding upon the board as if signed by the president thereof. Whenever the president of the board desires to designate a person to affix the signature of the president to any bond, warrant, certificate, contract or any other written instrument, he shall send a written notice to the board containing the name of the person he has selected and a designation of the instrument or instruments such person shall have authority to sign. Attached to the notice shall be the written signature of the president of the board, executed by the person so designated, with the signature of the person so designated underneath. The notice shall be filed with the secretary and presented at the next meeting of the board for its approval and shall be printed in its proceedings.