The board of supervisors at the time of making the levy of taxes for county purposes shall levy a tax for that year upon the taxable property in the district for the interest and redemption of district bonds. Such tax shall not be less than sufficient to pay the interest of the bonds for that year and the portion of the principal due or to become due during the year, and in any event shall be sufficient to raise annually for the first half of the term of the bonds the sum necessary to pay the interest thereon; and during the balance of the term, sufficient to pay the annual interest and to pay annually a proportion of the principal of the bonds equal to a sum produced by taking the whole amount of the bonds outstanding and dividing it by the number of years the bonds then have to run. All money so collected shall be paid into the county treasury to the credit of the district bond retirement fund and be used for the payment of the principal and interest on the bonds and for no other purpose until all bonded indebtedness of the district has been paid in full. The principal and interest on bonds shall be paid by the county treasurer upon the warrant of the county auditor out of the district bond retirement fund if that fund has sufficient moneys and otherwise out of any other funds of the district. The county auditor shall cancel and retain such bonds and coupons when he draws his warrants on the treasurer in favor of the owners thereof.

(Enacted by Stats. 1935, Ch. 389.)