The bond tax shall be sufficient to pay the interest on the bonds for the year and the portion of the principal becoming due during the year. The bond tax shall also be sufficient to raise annually for the first half of the term of the bonds a sum sufficient to pay the interest for that period, and, during the balance of the term, sufficient to pay the annual interest and to pay annually a proportion of the principal equal to the amount produced by dividing the total amount of outstanding bonds by the number of years the bonds then have to run.