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When you need a really good vacuum, and you need it to last a long time, the concept of "air tight" takes on a whole new meaning. For example, glass leaks. Not the seals or joints, but the glass itself. Radio tubes, discharge lamps, and similar applications require a level of perfection in their vacuum and/or inert gas atmospheres that cannot be achieved by any practical container: No matter what you make it out of, it's going to leak too much.
The solution is to place inside the sealed chamber a substance, known as a "getter" that absorbs stray traces of moisture, oxygen, or other undesirable contamination that works its way through the glass or metal. Since only a small amount of impurity is going to work its way through, a small amount of getter can keep the system clear for the lifetime of the product. (The getter also clears out any contamination you may have introduced at the beginning, making manufacturing perfection less important.)
Barium is a common getter, and this barium getter wire was manufactured in 1941 for use in radio tubes, and has remained sealed in an unopened vacuum can ever since. My theory is that it's because the opening key (not visible in the photos) apparently broke off when someone tried to open it (there is a residual stem where it was attached to the lid).
I guess one thing we can be sure of is that the vacuum in that can is really, really clean.
Source: Ethan Currens
Contributor: Ethan Currens
Acquired: 30 October, 2006
Text Updated: 19 November, 2007