As soon as convenient after the passage of this act the common council in being of said city shall divide the territory of said city into wards, the number of which shall be determined by the common council, but there shall not be less than three wards, which said wards shall be as nearly equal in population as possible, and each shall consist of compact territory and have a population of not less than five hundred people; the number and limits of which said wards shall remain until otherwise changed by the common council of said city, and a record and map of said wards shall be entered upon the journal of the common council of the said city and preserved. No change shall be made in the number or the limits of the said wards within six months previous to any regular election to be held for the election of city officers; provided, however, that should additional territory be taken into the corporate limits of said city within said six months, such additional territory may by the common council be added to existing and contiguous ward or wards or made a separate ward or wards.