Question 26602
You can do this in 3 or less weighings.

Weighing 1) Separate the coins into two equal piles of 6 coins each and put the left-over coin aside and place the two piles on the balance with 6 coins on each side.

If the scale balances, the heavy coin is the one you put aside.

Weighing 2) If the scale doesn't balance, take the six coins from the heavier side and divide them into two equal piles of three coins each and put the left-over coin aside. Now weigh the two 3-coin piles.
If the scale balances, the heavy coin is the one you put aside.

Weighing 3) If the scale doesn't balance, take the three coins from the heavier side and divide them into two equal piles of 1 coin each and put the third coin aside.
Now weigh the two 1-coin piles. If the scale balances, the heavy coin is the one you put aside.
If the scale doesn't balance, the scale will indicate the heavier coin.