Here is the Tesla's electron gun:
http://en.wikipedia.org/wiki/File:Sp_xmtr.png

"In April 1887, Tesla began investigating what would later be called X-rays
" We now know that this device operated by emitting electrons from the
single electrode through a combination of field electron emission and
thermionic emission. Once liberated, electrons are strongly repelled by the
high electric field near the electrode during negative voltage peaks from
the oscillating HV output of the Tesla Coil, generating X rays as they
collide with the glass envelope. " From:
http://en.wikipedia.org/?title=Nikola_Tesla

So we know that the electrons are liberated from the end of antenna.
But where the electrons come from?
From: "the earth"?
S*