05-08-2014, 09:32 AM
(05-08-2014, 01:16 AM)Alan Douglas Wrote: I presume the composition material is absorbing moisture. Metal and glass are impervious to moisture but no plastic (or wax) is, certainly not phenolic resins if that's what was used.
It's tricky because you'd think that moisture absorption would lower the resistance, not raise it. Likewise, if the fillers are hygroscopic - as some were - then again, surely the resistance would fall? Some people have heated resistors in an oven and seen a reduction in resistance - although this proves to be short-lived in practice - but you can see how this could add weight to the hygroscopic theory...
Personally, I think there must be some other effect. I'm willing to believe that moisture absorption could be part of it, but I suspect that changes in contact area between the carbon granules is a factor as well. I could well imagine that happens after the initial compression during manufacturer - one theory suggests that the granules "relax" slightly, and I could understand that heat could contribute to this in service.
As I say, I don't have the answers, and I don't think anyone outside of the resistor industry does. Searching reveals nothing but long threads of speculation and anecdotal evidence. I did find a link to a promising IEEE paper, but it's not a free download: http://ieeexplore.ieee.org/xpl/login.jsp...%3D1134810
