§ 89-i. Complaints as to price of water; investigation by commission; forms of complaints. Upon the complaint in writing of the mayor of a city, the trustees of a village or the town board of a town in which a person or corporation is authorized to supply or distribute water for domestic, commercial or public uses, or upon the complaint in writing of not less than twenty-five customers or purchasers of such water in such municipality or upon complaint of a water-works corporation supplying such water, as to the rates, charges or classifications of service for water sold and delivered in such municipality, or upon the complaint in writing of the department of state, or as to the methods employed in furnishing such service, the commission shall investigate as to the cause of such complaint. When such complaint is made, the commission may, by its agents, examiners and inspectors, inspect the works, system, plant, devices, appliances and methods used by such water-works corporation in supplying and distributing such water, and may examine or cause to be examined the books and papers of such water-works corporation pertaining to the supplying and distributing of such water. The form and contents of complaints made as provided in this section shall be prescribed by the commission. Such complaints shall be signed by the officers, or by the customers, purchasers or subscribers making them, who must add to their signatures their places of residence, by street and number, if any.