[Boatanchors] The black silver contacts

[Garey Barrell]

Well, actually silver sulfide IS conductive.  It's not as conductive as 
pure silver or coin silver, but it IS conductive.  In high power 
handling service, contacts with silver sulfide formations tend to heat 
from the  _reduced_  conductivity of silver sulfide, causing corrosion 
of the underlying copper which increases the resistance, which increases 
the heating, which...  you get the idea.

What makes silver sulfide essentially inconsequential in rotary switches 
in our gear is that it is also very soft, and easily displaced by the 
wiping action of MOST contacts.  Relays with silver contacts are 
typically designed to "wipe" by making the support arm flexible, causing 
the contacts to slide against each other, again displacing the sulfide 
and exposing the clean silver.  Close examination of a rotary switch 
after "exercise" will show a bright silver line where the spring 
contacts have displaced the sulfide.  If the "line" doesn't appear, it 
indicates that the fixed contacts of the switch have been "sprung", 
(possibly by someone "cleaning" the switch with tools!), and are no 
longer making reliable contact, regardless of the presence of sulfides.

So I stick with my contention that except in the most extreme cases, 
(storage in a barn or wet basement,) spray cleaners or attempting to 
clean with Q-Tips is far more likely to damage the switch than "fix" it.

An interesting side ...   The explosion that brought down TWA flight 800 
a few years ago is suspected to have been caused by  _CONDUCTIVE_  
silver sulfides formed on silver hardware inside the fuel tank.

73, Garey - K4OAH
Atlanta

Drake B & C-Line Service CDs
<http://www.k4oah.com>



Peter Burbank wrote:

> Hi gang,
> Just as many of us have suspected for years, the notorious "black 
> stuff" that makes bandswitches
> intermittent and the roller coil in your tuner crackle and spark is 
> not conductive. First of all it's
> not silver oxide but silver sulfide. Google strikes again and here it 
> is in writing from the people
> who seem to know and see all......the insurance industry.
>
> From the "Container Handbook" "Cargo Loss Prevention Information from 
> German marine insurers."
> "Hydrogen Sulfide reacts with silver to form silver sulfide (AG2S) 
> which results in the formation of
> a black film of insulating sulfides on the surface of silvered circuit 
> contacts and relays and on
> silvered conductors (e.g. coils)."
> Does any one know of a contact cleaner that removes the black film? 
> The best cleaner I ever used
> was the red "Contactene" sold by GC in the '50s but it was banned due 
> to carbon tet content.
> 73
> Pete NV4V
>
>