DW> The electrons are travelling in a closed path - part of which
 DW> is through the vacuum in the tube, and the rest is through
 DW> various wires, the power supply, etc.. The net momentum of
 DW> the electrons is zero, since the momentum of the electrons in
 DW> part of the path is balanced by that of those on the opposite
 DW> side of the loop.

The total momentum of the electron-apparatus system is constant (call it
zero w.l.o.g.).

 DW> If electrons lose some momentum by collisions with nuclei, a
 DW> counterbalancing force must be applied to electrons elsewhere
 DW> to keep the current going. Basically, this is what happens in
 DW> the power supply.

Right.  I'd forgotten about including the power supply in the system.  It
is, after all, an essential component!

---