Section 61.   FUNDS TO BE KEPT.
   There shall be maintained in the treasury the following funds for the support of which the council may levy taxes:
   (a)   A sinking fund for the purchase, or payment when due, of any bonds or any debt of the city and to pay the interest on all bonds and other obligations of the city. The council shall Levy an annual tax sufficient to meet all obligations against this fund when due, unless otherwise provided for.
   (b)   A public utility fund or funds for the acquisition, construction, extension, maintenance, and operation of any public utility owned and operated by the city, including the payment of the interest on any bonds or other indebtedness which may be a lien upon such utility. There shall be paid into this fund all moneys derived from the sale of bonds issued on account of any such utility, and from the operation of such utility, and from the sale of any property acquired for or used in connection with, any such utility. There shall be paid out of this fund the cost of the purchase, construction, extension, operation, maintenance, and repair of such utility, including the interest upon all bonds or other indebtedness which may be a lien upon such utility. Any surplus in said fund may be used for the purchase of any bonds or certificates of indebtedness issued against said utility, and for the payment of such bonds or other indebtedness upon their maturity. Separate funds and accounts shall be kept for each such utility operated separately, and in case two or more utilities are operated together the funds and accounts shall be kept separate as far as practicable.
   (c)   A general fund for the support of such other funds and for payment of such expenses of the city as council may deem proper. Into this fund shall be paid all moneys not herein provided to be paid into any other fund.
   (d)   A permanent improvement revolving fund. There shall be paid into this fund moneys received from special assessments levied for local improvements. The council may be resolution determine the aggregate amount of the assessments for local improvements which, in its judgment, shall be extended for payment, as is provided in this charter. The council may order the issuance and sale of bonds representing such aggregate sum which shall entitle the holder thereof to demand and receive from the city of Gaylord upon the surrender of such bonds to the treasurer on or after the date of payment thereof, the amount of money named therein to be paid, with the date of interest stipulated to be paid thereon to the due date thereof and not after such date. Such bonds may be issued in such amounts and become due on such dates as the council may determine. The proceeds of the sale of said bonds shall be paid into the permanent improvement revolving fund. The council may, in its descretion, either sell said bonds direct to investors, or may contract for the sale of all such bonds that may be issued during the calendar year. No sale of such bonds by contract, shall be made except after advertising for bids, at least one week prior to sale and such sales shall be made to the lowest responsible bidder. Bids must be asked on the basis of a rate of interest specified in the proposals and on the net interest basis on which the bidder will pay par for the same.