Brain warm-up C (Counterfeit coins)

[EvilAndrew]

For the start of the semester, I have decided to give everyone a quick brain warm-up. Feel free to e-mail me your solutions (my SIUE address starts with â€Ã...“rlamoni@â€Ã,) I will post a reply to this message with the names (or pseudonyms if desired) of the correct submitters in a day or two or when a sufficient number of people have answered the question. At that point, I will add a new question. Each question will have a title that could be a clue or just an inside joke. ;-)

Please DO NOT reply to the tread with an answer to the problem. E-mail it instead.

[EvilAndrew]

   You are given eight stacks of coins, a pointer scale (one that tells the weight) and the following knowledge.  Figure out which stack of coins is counterfeit in the minimal number of weighings and explain the procedure used.

1) You know that all of the coins in one of the stacks are counterfeit.
2) You know that all the coins in the other seven stacks are genuine.
3) You know that each stack has seven coins.
4) You know the exact weight of a genuine coin.
5) You know that each counterfeit coin weighs 1 gram more then a genuine coin.

[EvilAndrew]

I have received five answers to this problem.  Three of them are exactly what I was looking for.  The other two, however, fell for my trap. ;-) For reasons of fairness, I am now going to give a little hint.

I am glad to see that everyone remembers the concept of binary search.  I selected the number 8 for the coin stacks specifically to suggest that concept.  However, a simple balance scale[Wikipedia] (the kind that just compares masses instead of providing weight measurements) would have been ideal for the task of binary searching.  Since I provided a pointer scale[Wikipedia] I am looking for you to take advantage of it.

I will conclude this puzzle tomorrow at noon.  I will be posting problem D, which is a general knowledge problem, sometime before then.

[EvilAndrew]

Solvers:
1. Pete Lamonica (solved: 1-13-2005 10:23am)
2. tom von kercey (solved: 1-13-2005 1:34pm)
3. The Killer Llama (solved: 1-13-2005 2:03pm)

Answer:
Only 1 weighing is necessary if you, first, set the stacks down in a row and name them stack zero through seven.  Then you weigh a single group of coins comprised of 0 coins from stack 0, 1 coin from stack 1, 2 from 2, 3 from 3, and so on.  The total extra weight in grams (above 28*[genuine coin weight]) is the counterfeit stacks number.