I found this in the attic of a house built in 1908. The black wires on the left went outside through the soffit. Most of the house still had active knob & tube wiring. My voltage sniffer indicated that this fixture was ‘hot’.
Looks like an old communications system protector for the phone line.
Lightning arrester(telephone)
Yes, thank you. I went on Google with your idea and an image came up exactly like the one I found.
Yeah some kind of solenoid.
It’s not a solenoid.
Ooops that was a blurb.
Too much multitasking today.
surge arrester is designed to limit surge voltage by discharging the surge current to ground.
This is what is under that round thingy.
i.e. spark gap
I didn’t want to get too technical for you Bob 