All corporations, co-partnerships, and persons exercising franchises in the city shall give courteous, efficient, and adequate service at reasonable rates. A reasonable rate shall be construed to be one which will, with efficient management, normally yield, above all operating expenses and depreciation, a fair return upon all money honestly and efficiently invested in the plant and equipment used by the company in the public service within the city. This shall not be construed as a guarantee of a return and in no case shall there be any return upon the franchise value. Within those limits, the determination of the maximum price or rate to be charged by any company for service rendered to the city or to any person within the city shall, be made, if possible, by direct negotiations between the company and the council at public hearings. In case of failure to reach an agreement by this method, the council shall, not less than thirty days before the expiration of any existing rate schedule or agreement, appoint some expert as its representative; the company shall appoint a representative; and these two shall by mutual agreement select a third person, preferably an expert in valuation and rate-making, who shall together constitute a board of arbitration. This board shall report its findings as soon as possible and the rate which it shall agree upon by a majority vote shall be the legal rate, subject to revision by any court of competent jurisdiction. Schedules of rates thus fixed shall be as flexible as may be, and shall in no case fix a definite rate for a period of more than five years. The city and the company, may, by mutual agreement, revise existing schedules of rates at any time, proceeding in each case as provided for the original fixing of the rates.