Find the Bogus Coin

RAY: Imagine that you have in front of you fifty coins. They all look exactly alike except one of them is a fake. Because it's a fake, it weighs a couple of grams more than a real coin. So, if you had a balance scale, and you knew which was the bogus coin, you would put it on one side of the scale, a good coin on the other side...

TOM: ...and it would be immediately obvious from this imbalance which was the phony coin, because it's heavier than a real coin.

RAY: Right. Knowing that, you have in front of you fifty coins -- one of which is bogus. The question is, what is the fewest number of weighings on a balance scale,that you need to perform, to determine which coin is bogus?

TOM: And, Part B of the puzzler: How can you do it in four?

Think you know? Drop Ray a note!

[ Car Talk Puzzler ]