after 7 weighings

1.Devide the balls into 3 piles(729 balls) and weigh 2 piles, we can find the heavier pile.

2. Then devide the 729 balls into 3 piles(243 balls) and weigh 2 piles, we can find the heavier pile.

3. Then devide the 243 balls into 3 piles(81 balls) and weigh 2 piles, we can find the heavier pile.

4. Then devide the 81 balls into 3 piles(27 balls) and weigh 2 piles, we can find the heavier pile.

5. Then devide the 27 balls into 3 piles(9 balls) and weigh 2 piles, we can find the heavier pile.

6. Then devide the 9 balls into 3 piles(3 balls) and weigh 2 piles, we can find the heavier pile.

7. Weigh 2 balls of the 3 balls. We can find one hevier ball.

Excuse my poor English.