As Bill Jepson explained, not quite the same thing. By heating the filament you get a cloud of free electrons released from the filament. They are the initially accelerated (and focused) a few inches, the really high voltage then attracts them and further accelerates them to make them hit the fluorescent layer(s) on the front glass. You don't want the electrons to hit air molecules on their way and scattering them (or forming an ionization trail creating a shortcut), thus a vacuum tube.

Coming to think of it, can you actually get a spark in a perfect vacuum? Is a spark and ionization of the air (or any gas) the same thing?

Finn

Denny wrote:

This is very interesting and makes sense since in a TV picture tube electrons are forced to jump a gap in a vacuum of a foot or more.

Denny

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Finn Lassen
Sent: Monday, September 05, 2005 9:02 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Another great flying day = another day of troubleshooting

Hmm... and here I thought that vacumn was a better insulator than air ...
Why is the air pumped out of lightbulbs?

Finn

rijakits wrote:

Not exactly the same, but running the direstion:

During WW II the first fighters didn't run pressurized ignition harnesses.

Tales from Gruman state that a couple of test pilots encountered a sudden "Ignition failure" once they reached around 36 K feet.