Section 100. The mayor shall be recognized as the official head of the city for all ceremonial purposes and shall be recognized by the courts for the purpose of serving civil process and by the governor for military purposes. In time of public danger or emergency, as determined by the city council, he may, with its consent, take command of the police, maintain order and enforce the laws; and he shall have all the authority and powers conferred upon mayors by sections eighteen and nineteen of chapter thirty-three. He shall be chairman of the city council and chairman of the school committee. He shall have no power to veto but shall have the same powers as any other member of either such body to vote upon all measures coming before it. He shall perform such other duties consistent with his office and with sections ninety-three to one hundred and sixteen, inclusive, as may be imposed upon him by the city council. During the absence or disability of the mayor, or during the time such office is vacant, his duties shall be performed by the vice-chairman. In case, at any time, there shall be neither a mayor nor a vice-chairman, the member of the council senior in length of service, or, if more than one have so served, then the member senior both in age and length of service shall perform the duties of mayor until a new mayor has qualified. The mayor shall have no power of appointment, except of the employees mentioned in section twenty-five and except as provided in section one hundred and two.