|Radioactive spark plugs.
For some crazy reason, in the 1950's Firestone made automotive spark plugs containing radioactive polonium. Presumably the idea was that the ionizing radiation would allow the spark to travel more easily, making for better ignition. I think it's a fairly far-fetched idea.
According to this excellent book, they contain(ed) Polonium-210, with a half-life of only 138 days (the book has further information on pages 79 and 100 if you're interested). Whatever radioactivity there was in 1950 is long gone now! Though, my Geiger counter does reveal a very slight increase over background, maybe 300cpm, around the ceramic portion of the plug, none around the electrode tip. My guess is there is some totally unrelated radioactive contamination in the ceramic material or the glaze.
Amusingly, the Geiger counter I used to test them is a very nice one I got (for $40) at the closing out auction of a local Bridgestone/Firestone tire plant (it's the one that made the tires that practically put Firestone out of business in 2001). So I'm using equipment from a failed Firestone plant to test a failed Firestone product from 50 years ago!
This, incidentally, is the 100th sample installed in the table.
One spark plug is in the table, and the box containing the remaining eleven I have are in the Hot Box, because even though no actual radioactivity remains, the box proudly proclaims that they are radioactive (I guess people really went for that kind of thing in the '50s).
Here's an interesting article about different elements in spark plugs.
Source: eBay seller glenben1
Contributor: Theodore Gray
Acquired: 17 July, 2002
Text Updated: 18 January, 2009
Sample Group: Spark Plugs