Mercury relay

[ppppenguin]

I was sorting through some boxes of junk and found this mercury relay. I reckon I've had it since I was a kid in the early 1960s. The label is obviously German. It's possible it came from East Germany since we knew some people from there - they were professors of English and came to the UK to keep up to date -  and they may well have brought it as a present for me.

Otherwise I know nothing about it. I have no recollection of applying power to the coil though I surely must have done at some point.

There is a simple black bakelite cover.

[PerdioPal]

looks like a displacement type where a lump of iron displaces the mercury to close the contacts, very quick closure and no contact bounce. They are used in EMP (electromagnetic pulse, from nukes!) testing , loads of volts/amps all of a sudden. Albeit of a larger size.

[Kalee20]

I came across similar relays, used (why?) to switch input power to a Westinghouse 3-phase transformer-rectifier unit supplying DC to cinema arc lamps. The high-current path was kept local, and the low-current switching was beside the projectors.

There would appear to be no advantage in mercury relays - the control cabinet even had a couple of spare relays, so they were not considered faultless by design. The number of switching operations was three per hour (for 20-minute reels), hardly massive... However, it did introduce me to a bit of new technology!

Unfortunately the rectifiers themselves, despite dating from 1936, were not MAR's but metal (whether copper-oxide or selenium, I never found out!)

[Mike Watterson]

The only time I ever encountered a mercury relay was on a Telex interface in the BBC. It looked like mains electrolytic.. Very occasionally the receive only telex stopped (it was of the kind without a ribbon using special paper) and the relay had to be unplugged and shaken? It's about 50 years ago.

No email.

[PerdioPal]

"There would appear to be no advantage in mercury relays" Not no advantage I think but very few, good ones at that too. To name the ones I can think of...
No contact wear
No contact bounce
Very little thermal EMF
When I was at Vitramon we used hundreds of mercury wetted reed relays to switch the test gear to the capacitance measurement instrument (they were very expensive HP jobs measuring 100 or so per second). These relays had to be a certain way up for use.
Then you have the most ingenious time delay tilt switches used in clocks etc. that had a restriction to slow the reverse flow of the mercury, very clever.

[djrm]

Then you have the most ingenious time delay tilt switches used in clocks etc. that had a restriction to slow the reverse flow of the mercury, very clever.

Here is a brief video of my Gents bellringer mercury delay in operation: https://youtube.com/shorts/OPXZ7pls_zU
After being tilted by the 5 minute mechanism then the bell rings until the mercury runs through the constriction into the other chamber.
   
David

[Kalee20]

"There would appear to be no advantage in mercury relays" 

That's quoting me out of context! 

No advantage in the particular application I described - switching 440V utility power (in fact the plunger of one used to stick sometimes, needing to switch on and off again to shock it free).

But I agree with the others - no contact wear; no bounce; predictable contact resistance (you didn't include no wetting voltage either, so they're suitable for microvolt level signals).

[DrStrangelove]

I recall shedloads of mercury wetted reed relays in Fairchild 4000 semiconductor automatic test equipment.

This sort of thing, though much modified for nA leakage testing and sans the dynamic test part:

https://www.chiphistory.org/77-fairchild...est-system

IIRC they were still used until 1985 or 1986 after which a modern LTX tester was installed.