On Friday 07 October 2016 11:48:07 andy pugh wrote: > On 7 October 2016 at 15:47, Gene Heskett <ghesk...@shentel.net> wrote: > >> I have never found aluminium to be non-conductive. > > > > Its a good conductor, IF you can achieve a 'gas tight' connection, > > AND maintain it for decades. > > No, really. Get some aluminium and your multimeter. You will find that > it conducts.
Force one probe to actually dent the surface so you know you are making a connection WITH that probe. Then lay, or let gently swing to touch a clean looking spot, the other probe, without using the point, or a sharp edge on the test aluminum and the gentlest touch you can manage. Your digital volt/ohm meter, which likely doesn't use more than its internal battery voltage, 9 volts max, shouldn't be able to detect a connection. You will not get a connection until you have applied enough force to crush/penetrate that oxide film, which if its fresh, as in a 1 hour old machined surface might be 10 atoms thick, but its still growing thicker, maybe 100 atoms thick 2 weeks later. Thats good for around 400 volts before a single electron can penetrate it by a microscopic arcing. Its also the near top of the list of electrical insulators, I think only the silicone dioxide used as the gate insulation layer in a field (mosfet,hexfet) effect transistor is above it. That 1 hour old machined surface is still rough enough, and that roughness concentrates your test force to the tip tops of that roughness, so relatively little force will make contact as the oxide film is now mechanically damaged. Arrange to leave the probe laying there for a few hours. Because air can get under the probe, the damage will be repaired, and the connection will disappear unless sufficient force is applied, and stays applied so that the oxygen in the air is forced out of the contact and cannot over time degrade it. This is then a "gas tight" connection, and many millions have been spent designing an electrical box screw the top bolt in to grip the joint and make a good connection today, and because it has the right springiness, is still applying enough pressure years later to maintain that gas tight joint in a gas tight category. Its a very elusive target, and the electrical people are told to cover the end of the wire with a grease called no-ox before they stick it into the terminal and tighten the screw. I've seen union electricians that had a tube of it in the kit, maybe 8 oz ten years ago, still has 6 oz in it a decade later. I'd encourage them to find another line of work. They are a long term liability on the jobsite. OTOH, I've seen them stop work and go get another tube of it when they run out. Its job is to seal the air out of a joint, thereby preventing this failure. In many locales it is illegal to use alu wire without this grease. In the stricter locales, alu wire is illegal if the building is a dwelling. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
