Can a purely thermonic cathod work?

[hal]

In each and every vacumn tube I have ever seen the cathod is connected 
to a source of
current.  Therefore, I reason, the current must enter the material that 
makes up the
cathode and launch the electrons that make it up toward to anode or 
grid(s). The heater
must, likewise, raise the temperature of the cathode to make this 
possible since a cold
cathode would never emit anything.

But wait!   It this really the case, I wonder?   We were all thought to 
think of electrical
circuits through their hydrolic analogy -- wires as pipes, a battery for 
a pump, a switch
for a valve.  Gravity acted as resistace.  While successful for some 
circuits it might have
sealed in the wrong ideas for an electron tube. 

What about a solid state theory of the cathode?   Here a cathode might 
be thought of as
a material that when heated boiled electrons out of its metal.  Once 
free the electrons are
attracted to the grid(s) and plate and begin the trip to those 
elements.  The cathode is not
a sprinkler supplied by the batteries but a metal that showers off 
electrons when heated.

If this makes sense then you should be able take any electron tube and 
re-engineer it so
the cathode uses a purely thermal energy source w/o ever connecting it 
to a external current source. The tube should work just fine.

--h.