If any excess of the principal sum of the proceeds of any issue of bonds shall remain unexpended after the full accomplishment of the purpose for which such bonds were issued the board, by resolution, may direct that the same shall be applied to the purchase of bonds of that issue, or to the payment of any such bonds, at maturity or on redemption, or to the payment of interest thereon or of premiums due on the redemption thereof, or that it shall be transferred to the revenues pertaining to the electric system in connection with which such bonds were issued.