It looks like I've stumbled on a working
unsaturated Weston standard cell.
I picked up a couple of Solartron DVMs with the giveaway "CAL 1.019" and "CAL 1.0185" buttons and trimpots. After being on for a while, and twiddling the trimpot so the display matched that of my Agilent 34410A, one read 0.04 (i.e. clearly dead), and the other 1.019. What? It isn't dead?!
I'll ignore the dead one, and concentrate on the apparently good one.
It is inside a Solartron 1420.2BA. A datasheet is stamped "received 2 Feb 1966". The service manual states "internal Weston standard cell (unsaturated)" and "Weston Cell Temperature Coefficient +-0.0005% per degree C", but does not state the specific type. There are no ICs nor valves to hint at manufacturing date; the
datasheet even lists the number and type of diodes and transistors! However, the nixie displays are marked 69xx, so it looks like the meter is ~1970 vintage, i.e. over 50 years old.
When spelunking, I used my Agilent 34410A on Hi-Z setting to measure the standard cell voltage, and it was 1.018725V when the unconditioned room temperature was 25C. According to
Conrad Hoffmann, the voltage should be 1.0190V @ 20C, decreasing by 5µV/C as the temperature rises, so if "new" my cell should be 1.018975V. Cells tend to decrease voltage by 25µV/year (with quite a wide range), so my 250µV fall equates to 10years. Not bad for something with a 7-18 year lifespan.
The cell is inside the black plastic case in the centre of this picture; I can't see any identifying information and it looks like it is simply a box to protect the glass phial. I'm not going to attempt to desolder it to read the other side.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1514887)