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The evacuation tips of these tube are the narrow points at the top.
The evacuation tip is a port on any glass envelope or vessel inside of which specific gasses or a vacuum must be held. They are very often seen on nixies or vacuum tubes. It is used to evacuate gases from the tube, and in the case of nixies, add the proper mix of gases before being sealed. It provides a convenient port for the amateur glasshacker to introduce gases to alter the performance of the tube. It is also known as the "tubulation" or "pip".
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Base of an Octal tube with the spigot housing the downward-pointing evacuation tip.