Vacuum-tube speed

[Göran Axelsson]

bfranchuk at jetnet.ab.ca wrote:
> der Mouse wrote:
>> That brings up something I've occasionally wondered about: how fast do
>> electrons move in a vacuum tube?  (I'm talking about the free-flight
>> path between cathode and plate.)  In particular, what's the time delay
>> between emission from the cathode and reception by the plate?  I
>> imagine it depends on the plate voltage; does that make enough of a
>> difference to care about?  I don't remember enough of the constants to
>> figure out what sort of acceleration a gradient of, say, 200 V/cm
>> imparts to a free electron....
>>   
> I think you need study a broken tube to find the distance
> from the cathode first.

The final speed of an accelerated electron only depends on voltage, not 
the distance. A quick calculation gives the final speed close to 2% of 
the speed of light at 200 V acceleration. That means it travels 10 cm in 
aproximately 30 ns. (not counting relativistic effects and putting mean 
speed to 1% of c)

In all practical cases the real distance is much smaller than that, 
tubes usually isn't so big, at least not in a digital circuit. The x-ray 
tube I have is 50 cm, but that is a totally different story.   :-)

/Göran