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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    \$\begingroup\$ Is this side up for orientation or right angles? \$\endgroup\$ Feb 9, 2018 at 0:26

3 Answers 3


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\$
    – user105652
    Feb 9, 2018 at 2:07
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    \$\begingroup\$ It might be used as overvoltage protection. \$\endgroup\$
    – mpy
    Feb 9, 2018 at 11:28

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


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, 2018 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, 2018 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, 2018 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, 2018 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\$ Feb 9, 2018 at 21:12

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