3?
1) Take 7 bars in each scale. The scale which is lighter must continue the light bar. If they are in balance --> see my first answer.
2) Take 3 bars of the remaining 7 in each scale. The scale which is lighter must continue the light bar. If they are in balance the 7th one (not weighed) must be the light one.
3) Take 1 bar of the remaining 3 in each scale. Either the scales are in balance, then the 3rd one must be the light one, or they are not in balance, then you can also identify the lighter one.
I want a prize now for translating this stuff into English!!!