>What does a Neon bulb normally do when connected to DC (I didn't think to
>try it until just now, but I don't have one handy)? Does only one electrode
>glow?

  Yes. Turn it around and the other electrode will be the one
  that glows.

  Not sure what is causing only one to glow on AC. Maybe the ignition
  voltage is higher on one electrode than the other? See what happens
  when you reverse it.

  Reg Neale