-> I think we have to consider the power consumed by the tube. Obviously  
-> ignoring the filament, we need to consider if the tube dissipates  
-> energy in an unsymmetrical manner and this we assume mostly at the  
-> plate electrode. Since we know there is a loss of energy there then,  
-> it is being emitted on one side mostly and depending on the physical  
-> shape (not a cylinder) it might result in a net motive force. Perhaps,  
-> it isn't very great but it's still something.  
->   
->  M*i*k*e  
  
Yes. If the device is radiating energy asymetrically, then there will 
be some (tiny) reaction force. 
  
However, in most vacuum tubes, the basic design is cylindrical, with 
the heated cathode running down the axis, then the various "grids" 
suspended cylindrically around it, and the anode "plate" around the 
outside. Such a device would radiate energy pretty well symetrically, 
so it wouldn't produce a significant reaction force. 
  
Just for the sake of argument, suppose the device radiates 10 watts, 
all in one direction. So the force will be about 10/3e8, or 3e-8 
newtons. If the mass is, say, 0.01 kg, this will produce an 
acceleration of 3 micrometres per second-squared. After a day (about 
1e5 seconds), it will have picked up a speed of about 0.3 m/s, or about 
one foot per second. Hmmmm.... That's more than I would have guessed. 
  
                         dow 
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