The special tax for payment of bonded indebtedness authorized in Sections 1 and 2 hereof shall constitute a special fund pledged as security for the payment of such indebtedness. The special tax shall never be extended for any other purpose, nor collected for any greater length of time than necessary to retire such bonded indebtedness, except that tax receipts in excess of the amount required to retire the debt according to its terms may, subject to covenants entered into with the holders of the bonds, be pledged as security for the issuance of additional bonds if authorized by the voters. The tax for such additional bonds shall terminate within the time provided for the tax originally imposed. Upon retirement of the bonded indebtedness, any surplus tax collections which may have accumulated shall be transferred to the general funds of the municipality or county.