/biz/ - Business & Finance

I recently built myself one of these to test my silver coins. Thankfully, all of my coins "passed" this test, but many non-silver coins passed too - pre- and post-1982 pennies, for example. I knew that copper had similar diamagnetic properties to silver, but shouldn't a post-1982 penny with its 2% copper content just slide down with no resistance?

How can I be sure my silver isn't copper/brass on the inside, especially if I don't know what the exact dimension are supposed to be (i.e. an older bar or round)?