|Argon signal lamps.|
I'm guessing that these antique bulbs are filled with nearly pure argon, but I don't really know. Connected to a hundred or so volts they produce a characteristic purple-blue glow over the surface of the electrodes. Not enough to create usable light, but perfect as a signal light, the kind of thing that today would be done with an LED or neon indicator light.
Source: eBay seller ramblingguy
Contributor: Theodore Gray
Acquired: 28 February, 2009
Text Updated: 12 March, 2009