I've put a small battery powered commercial radio inside a Quality Street tin. The tin is all metal with rolled joints and the lid is a tight fit. I'd be willing to bet that there are no holes greater than 0.1mm around the lid's closure. It's a fairly common Sony ICFS22 FM/AM Pocket Radio with a telescopic aerial, and the tin is approximately 300mm in diameter.
Why does the radio work when receiving an AM station (1 MHz), but only produces static when switched to FM (100 MHz)?
Some further details of things I've done...
The tin IS magnetic, attracting a magnet well
The walls are 0.25mm thick (including paint)
The inside of the tin is unpainted
I grounded the tin to the earth wire from a UK mains electrical socket, and repeated the reception test. I filed off the paint where the wire touched the tin. Results:-
FM: Total static. No discernible change with /without earth wire.
AM: Tinny but recognisable radio station. No, repeat no discernible change in pitch or volume with /without earth wire. Surprised.
The tin with the ground wire...