high-resistance wire 40kOhm per meter

Carbon-core "ignition wire" is about 5K ohms per foot.

Tim N3QE

Tim,

yes, but the diameter is too big. I want to build several 3-axis sensors.

Michael

That'll be tough, there is not much of a market for the cable you want. Can't you make it from small snippets of regular very thin wire with, say, a 0201 size or smaller 402 ohm resistor every centimeter and slide it all into thin tubing?

Joerg,

That's a lot of work. At least the first meter from the sensor should have high resistance. That means I need 4 meter wire for a 3-axis sensor. And my idea was to have several 3-axis sensors mounted on a wood frame, so that field homogeneity can be measured simultaneously at all sensors. For example 3 x 3 x 3 = 27 sensors in a 1m^3 cube, each sensor is 3-axis, --> 81 sensors total. That's only possible if cheap high-resistance wire is available.

Michael

Then your only option might be what we had to do when looking for certified high voltage wire, call all the cable manufacturers that do custom runs. For example Tensolite in Florida. Talk to engineers there and if they can't provide the service they'll usually tell you where to call next. But once you find a source it will most likely not be cheap. With some luck you can partake in someone else's cable run and split the NRE.

Hobby or pro?

If it's hobby (and maybe even pro-lite), consider the notion of dipping string (or thread) in thin plastic goo with the right amount of carbon added. Then thread it through some heat shrink (or dip it in more goo without the carbon) for insulation.

What plastic? How much carbon? What string? I dunno -- but maybe this'll start some brain cells firing.

wet string?

Have you tried talking to NIST? It is their job to be friendly about these things.

Steve

Maybe try sewing thread dipped in Aquadag.

John

You can get 0.0031" OD nichrome wire at McMaster-Carr:

But you'll have to figure out the resistance per foot on your own.

Thinner wire is probably available - I just happened to know MMC has it off the top of my head.

Good Luck! Rich

Nichrome isn't going to work. Evanohm alloy has about 5 times the resistivity of nichrome, and 47 gauge Evanohm

Michael, this just dawned on me, you might want to call Isabellenhuette in Dillenburg, Germany (hey, I was in that town last March ...) and ask about their ISA-Ohm wire. Not quite the 40k but almost 17k for the really thin stuff:

As far as I know these guys build resistors with wire from Isabellenhuette:

Joerg,

That's very interesting, I think it's close enough to 40kOhm/m.

Thanks!

Michael

Be aware that the really thin stuff is .01 mm--that's 58 gauge. Wire that small floats on air when you break off a piece. It will be almost impossible to deal with.

Ignition wire, because of the high voltages and adverse environment, traditionally has enormously thick insulation on it. But the actual stuff inside isn't thick at all. The insulation provides a lot of support to the hokey "conductor" inside but the stuff inside does have some strength on its own. It's kinda like dirty twisted thread.

Terminating the carbon-impregnated-fiber-or-plastic conductors in ignition wire can be a real bitch, BTW. Soldering is a non-option. Crimping without insulation support pretty much guarantees that at the first flex it'll come apart. Maybe some skinny teflon tubing with the inner conductor threaded through would work OK.

I wonder if water with enough salts dissoved into it to reach kohms/ foot conductivity, inside skinny teflon tubing, would meet your needs?

Tim.

small

Yes, Michael will probably have to secure that in stiff plastic tubing. For initial tests a straw would work, those are cheap and easy to glue to stuff.

And don't sneeze before it's inside the straw ;-)

The easiest source for resistance wire is thermocouple alloys (like Constantan, for type J thermocouples, which was originally a low tempco resistor wire). My tables show 56 gage as the finest and that only has a tenth the resistance you want. It gets brittle (and becomes a fuse more than a wire) if you try to get thinner wire.

Some HV resistors are made with a buffer solution in vinyl tubing, you could get a roll of capillary tubing and mix up your own fill liquid.