How to Determine Counterfeit Coins with Only One Weighing

[AlexChandler]

There are seven stacks of coins, each consisting of 100 coins.
You know that real coins weigh 10 grams, but counterfeit coins weigh 11 grams.
You have an analytical scale that is accurate to .1 gram.
Each stack consists of either all real coins or all counterfeit coins, none of the stacks are mixed.
You do not know how many stacks are counterfeit or even if any of them are.
You want to determine how many stacks are counterfeit (if any), and precisely which ones are.
You want to do this in the least possible amount of weighings.

Whats the fewest number of weighings you need to make?
And how do you do it?

 

[Jonathan Scott]

Is there a catch? I make it 1.

Spoiler

Take 1, 2, 4, 8, 16, 32, 64 from each pile, measure the excess weight and convert it to a binary number

 

[AlexChandler]

Is there a catch? I make it 1.

Spoiler

Take 1, 2, 4, 8, 16, 32, 64 from each pile, measure the excess weight and convert it to a binary number

I'm impressed. :tongue: That was fast!