Morris: Golden Earring – Radar Love

Goose-pimple time - I LOVE  this songTrauma at home; my wife has just lost one of her golden earnings.  Fortunately I know someone who will sell me a bunch of similar earrings, for a price!

I know that one of his earrings is fake, but that’s okay because I can work out which one, replace my wife’s earring and still make a profit. 

I will have to bring out my earring weighing machine, the Radar Love, which lets me compare two groups of earrings and tells me whether one group is heavier or lighter than the other, or whether they are the same wieght.  This is the only clue I will get.

The Radar Love only has three charges.  At most how many earrings can I have bought and still be confident that I will find the fake?

Can you find a general formula for a given number of charges?

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