No public utility owned by the city whether acquired prior to the adoption of this charter or thereafter, shall be sold or otherwise disposed of by the city, unless the full terms of the proposition of said sale or other disposition thereof, together with the price to be paid therefor, shall have been embodied in any ordinance passed by a four-fifths vote of the council in the usual way, and submitted to the electors at a general or special election and approved by a majority vote of the electors voting thereon.