coin magnifiers

coin magnifiers
What device is used to prove the authenticity of the pieces? - No scale or lens ...?

A few years ago I sold some coins gold to a merchant. He tried the coins with a small electronic device. I think there was some electrical contacts in the slot. This was not a ladder .... What was the device? Who does? What is the principle of operation?

A company called igem produces relatively few different models of gold testers. These seem to be very popular. The question how this question is already answered on Yahoo Answers. I am including a link to this question and response and Web site link igem.