It shall be the duty of the Treasurer of said Village to keep separate accounts of all moneys received from the Electric Light Department and the rates and receipts of said Village for the use of electric light power shall be applied first to pay the interest as it becomes due upon the bonded or other indebtedness of said Village in connection with said electric light plant, and the excess, if any, shall be expended in the necessary repairs, management, and additions to said electric light plant. And if there remains unexpended a balance of the said electric light fund such balance shall be applied to pay the principal of the debt incurred on account of the said electric light plant if due and payable, or shall be set apart as a sinking fund, and shall be invested by the Treasurer of said Village with the approval of the electric light commissioners to be applied in payment of such electric light plant indebtedness when the same shall become due.