What is this?

[uslrs]

Hi All,  I’ve been cannibalising some old pcb’s and came across what I thought were four neons on a board with a couple of small coils and a variable resistor.  On closer inspection I saw there are two filaments which join at a small, central bead.  I then thought they might be some form of thermistor like the old RA53, used as a stabiliser in oscillator circuits, but I can’t find anything with four terminals like these. Despite the filaments being connected, the resistance between the two green leads is 180 ohms and 8 ohms between the red and black leads. There is no diode action between any pair and there is open circuit between red-green and black green. I’ve never seen bulbs that look like this, so any ideas as to what they are? 
Cheers.

[Gyro]

That's a heater/thermocouple - used for RMS to DC conversion - treat with care for now, they're hard to find. The horizontal wire (Green wires) is the heater and the other two the thermocouple.

The glass is evacuated (like the RA53) for maximum sensitivity, the heater should never be allowed to reach the point where it starts to glow.

Having more than one is a real bonus for you.  It means that you can use one for input and put the heater of a second one in a feedback loop from an opamp (comparing the outputs of the two thermocouples). This will linearize the output (the input current to thermocouple voltage output is square-law otherwise). The resulting DC current through the second heater should then be equivalent to the RMS current through the first one (depending on how closely they are matched). They will work well up to RF, but you can still calibrate the setup using a DC source.

Take a look at Linear Tech AN61 by Jim Williams, particularly Appendix A, to see how it was done in the HP3400A. It's a lot easier these days when you don't need neon opto choppers to implement a low drift opamp!  http://cds.linear.com/docs/en/application-note/an61fa.pdf

[uslrs]

Thanks Gyro - I guessed they might be something a bit special!  You learn something every day! I'll have a look on the web to see if I can find any specs, info or application circuits.

[Gyro]

Your welcome, I just edited my reply above with additional info (I had to break off earlier due to a planned power outage!).

[uslrs]

Many thanks for that extra info Chris - the AN61 is really helpful (and very interesting) - the arrangement of the pair in Annex 1 particularly. I found some basic info myself, so now got some serious playing to do!  The info I found on the web related to a heater thermocouple with a heater current rating of 5mA, so I did some measurements in that range and got the data I've attached as a graph.  I haven't been able to identify a manufacturer or part number, so have no idea what maximum heater current is likely to be.  Looking at the original pcb, it looks like the four were in some sort of series/parallel configuration, probably in some AC-DC conversion arrangement, as you suggested.  Thanks again for the help.

Les.