Has anyone used a bridge to measure wire?

[guitician]

I was wondering if, as an aid to using gauge/thickness, measuring with a Kelvin bridge could be used as a way of matching/comparing wires.

http://en.wikipedia.org/wiki/Kelvin_Double_bridge

#42 gauge has .5977 Feet Per Ohm @ 68 degrees F.

Or does the resistance vary too much along the full lenght to be of much use?

[salvarsan]

#42 gauge has .5977 Feet Per Ohm @ 68 degrees F.

Or does the resistance vary too much along the full length to be of much use?

My opinion is that resistance per 3500 feet is a more practical
measurement since that's roughly how much wire goes on a bobbin.

The industrial spec for magnet wire spec says:

AWG 42 can be 0.0025" diameter, +/- 0.0001",
which gives an acceptable resistance range of +/-8%.

For AWG 43 at 0.0020" +/-0.0001,
the resistance range is about +/-10%.

You can download the NEMA MW1000 magwire spec for free at:

http://www.nema.org/stds/mw1000.cfm/

This means that two otherwise Identical pickups, one wound from the
beginning of a 5lb spool, the other wound from the end of the spool,
will surely have different DC resistances.


-drh

[Joe Gwinn]

AWG 42 can be 0.0025" diameter, +/- 0.0001",
which gives an acceptable resistance range of +/-8%.

For AWG 43 at 0.0020" +/-0.0001,
the resistance range is about +/-10%.

To be pedantic, doesn't the resistance per meter vary with the square of the diameter of the bare wire?

#42: {(0.0025+0.0001)/(0.0025-0.0001)}^2= (1.08)^2= 1.17, or 17% total range.

#43: {(0.0020+0.0001)/(0.0020-0.0001)}^2= (1.1053)^2= 1.22, or 22% total range.

The variation is small enough that linear isn't a bad approximation.

If one has access to a 6.5-digit DMM, one can easily measure resistances to 0.0001 ohms, so it's practical to measure shorter (and more easily measured) lengths of wire, and compute wire diameter.

At 0.6 feet per ohm, a one foot sample would allow measurements of resistance of the length to one part in a thousand, so the accuracy of the resistance per length value would depend on how accurately one measured the length. A one meter sample with length measured to the millimeter would be practical.

As for variation of diameter with distance, the resistance method is no worse than a micrometer or optical comparitor.

[Joe Gwinn]

I was wondering if, as an aid to using gauge/thickness, measuring with a Kelvin bridge could be used as a way of matching/comparing wires.

http://en.wikipedia.org/wiki/Kelvin_Double_bridge

#42 gauge has .5977 Feet Per Ohm @ 68 degrees F.

A Kelvin Double Bridge will work quite well if you know the resistance of the low resistance reference resistance, which may be a problem.

One can buy used calibrated standard resistances quite cheaply these days, and measurement labs have gone digital.

The subject is beaten to death in "Methods of Measuring Electrical Resistance" By Edwin Fitch Northrup. A scan is available at Google.

One thing though. The oldtimers spent a lot of time worrying about thermoelectric effects, because they had to do everything with DC. Now days, it's far easier to use AC to drive the bridge, and use an earphone or DMM as the null detector.

[salvarsan]

To be pedantic, doesn't the resistance per meter vary with the square of the diameter of the bare wire?

#42: {(0.0025+0.0001)/(0.0025-0.0001)}^2= (1.08)^2= 1.17, or 17% total range.

#43: {(0.0020+0.0001)/(0.0020-0.0001)}^2= (1.1053)^2= 1.22, or 22% total range.

I see no significant conflict between our figures since mine also
derive from resistance calculations. I stated a median+/- rather than
a total range, and rounded off for simplicity.

Up though AWG 44, a manufacturor's quoted wire resistance values
derive from the diameter and not direct measurement, presuming a
standard copper resistivity instead of a measured resistivity for the
particular heat of copper in the wire.

In fact, the MW1000 spec describes insulated wire and its testing, but has
no bare wire resistance tables.

Conversely, AWG 45 and smaller are defined as ultra-fine and their
diameters are usually derived from resistance measurements.

In short, the best you can expect is that wire resistance will be within
+/-10% of the target because that's what may be practically manufactured.

A pickup maker's prime concern, sonic properties of insulated wire, is
seldom a concern of any manufacturor.

-drh

[Joe Gwinn]

I was wondering if, as an aid to using gauge/thickness, measuring with a Kelvin bridge could be used as a way of matching/comparing wires.

http://en.wikipedia.org/wiki/Kelvin_Double_bridge

Also look into "slide wire bridges", a fair description of how one variable element of the bridge is constructed.