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Might be super obvious, but I've not encountered one of whatever this little dude is before:

strangle little bulb-like thing

I figured some sort of vacuum tube, but no idea beyond that. The two poles aren't linked. Found inside a heating control panel. Help would be greatly appreciated!

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I think it is a neon lamp - possibly an NE-2. The "I3" designator on the PC board tends to confirm that - "I" would indicate "indicator".

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    \$\begingroup\$ That must have been a tough call Peter. \$\endgroup\$ – Sparky256 Feb 9 '18 at 2:07
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    \$\begingroup\$ It might be used as overvoltage protection. \$\endgroup\$ – mpy Feb 9 '18 at 11:28
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Neon lamp or gas filled surge arrester. Not likely a mercury tilt switch, since:
a) PCB mounted mercury switches are far more rare than neons/arresters
b) This looks like a PCB from a CRT monitor where gas filled arresters are very commonly used
c) There is no mercury bead visible

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On the right image you'll see there's some mercury at the bottom inside of the enclosure.

It is a mercury tilt sensor/switch.

enter image description here

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    \$\begingroup\$ That's just a weird reflection - I've given it a good eyeball and wiggled it about, definitely no mercury in there. Weird though, otherwise it looks identical to your pic. \$\endgroup\$ – slothgirl Feb 9 '18 at 0:08
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    \$\begingroup\$ I think that mercury you're seeing is a trick of the light. Looks more like a neon bulb to me. \$\endgroup\$ – brhans Feb 9 '18 at 0:10
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    \$\begingroup\$ They're sometimes used as sorta surge protectors in power supplies - so not intended to be seen. \$\endgroup\$ – brhans Feb 9 '18 at 0:15
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    \$\begingroup\$ In a mercury sensor – as shown in your picture – the contacts/wires are at one end of the tube, allowing the mercury to sit at the other end (in normal orientation). In the OP's picture, the contacts/wires go almost to the end of the tube (and the leads where they enter don't appear insulated). If it were a mercury switch, the mercury would be completing the circuit (virtually) all the time. Screams neon to me. \$\endgroup\$ – TripeHound Feb 9 '18 at 8:08
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    \$\begingroup\$ It's not a weird reflection and it's not mercury. It's part of the base of the fuseholder, behind the lamp, in that shot. \$\endgroup\$ – Brock Adams Feb 9 '18 at 21:12

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