[Radio Wrangler]

When they build a valve they include one or a few 'getters'

A getter is a grooved ring or cup shaped piece of metal. In the groove or in the cup is a plating of a highly reactive alkali metal such as barium with a protective cover of nickel plated over it.

The valve is evacuated through a glass tube - as much as a normal mechanical vacuum pump can do it RF heaters are used to warm the inner metal structure to help entrained gas escape and get pumped out. Then the tube is melted and sealed off. The vacuum in the valve still isn't good enough. Stronger RF heaters are focused on the getter ring/cups and they flash up to red heat. The top layer of nickel is evaporated off and condenses on the inner surface of the glass envelope of the valve. Then the barium boils off and condenses on top of the nickel on the glass envelope.

The exposed metal reacts with the reactive gas molecules, the inert gas tends to get anchored by adsorption rather than reaction. The pressure in the valve drops low enough now.

Sometimes the unfired ring gets called a getter, sometimes the metal deposit after a getter has been fired gets called a getter.

It's a sort of one-shot vacuum pump, though the reactive metal can keep mopping up the odd molecule for a very long time.

David