Hmm.... three leads, so it's probably a diode of some sort. They are so big b/c of the high voltage involved in transmitting; smaller tubes would probably break down and short.
Heck, just seeing that light up would make for an awesome wall decoration.
How about a mercury arc rectifier?
See here:
http://blog.makezine.com/archive/201...rs_are_aw.html
For size reference, see last image here:
http://www.subbrit.org.uk/sb-sites/s...er/index.shtml
