Accurately measuring diameter of very fine copper wire ?

Ummm... the wire gauge is a measure of the copper diameter (or more correctly, the cross sectional area), without the insulation. The added weight of the insulation coating is quite small when compared to the larger density of the copper. With flexible small diameter copper wire, there are a few additives, but they constitute a tiny percentage of the wire composition. My guess is the biggest error will be in the accuracy of the weight, not in the effects of the insulation.

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The former has to be demountable. I've made but not had the time to try winding yet. A platic binder spine warmed up and opened out to a flat bottomed V. Doubled up , end over end, and 2 cuts per side , so can easily cut the remainder after winding. Some thick PTFE wound around to decrease the width a bit to fit the trough, some thin elastic laid across , so under the wiring, to tie together in loops after winding. Well thats my theory. The original one probably failed because in the process of wrapping with tissue paper and adjusting to fit into the trough , the layup was seriously disturbed/ stressed, before lacquering.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Just a heads up that I emailed you per your request. My email as shown on usenet is munged.

Yes, you're probably right. I studied my supplier's reference chart some:

and it looks like single poly adds about 4% to the weight of 42 AWG, while a triple build would add nearly 10%. Not exactly negligible, but not enough to say that 42 triple could be mistaken for 41 single, by weight.

Still, when it comes to alternative approaches, I think measuring the resistance of a substantial length is going to be easier in terms of instrumentation precision than weighing it would be.

But, I've been happy with a good micrometer; it's a little quicker than measuring off a football field's worth of frail wire.

On Tue, 4 Aug 2009 10:38:19 -0400, N_Cook wrote (in article ):

I remember a "homebrew science" experiment I once saw where someone was able to do very precise measurements with a homemade apparatus that used the principle of a lever. If I recall, basically you put the thing you want to measure the thickness of under the end nearest the fulcrum and sheets of paper of a known thickness under the other. Knowing the length of the lever arms, the thickness of the sheets of paper, and the difference in the number of sheets which can be inserted with and without the object to be measured under the other end allows you to calculate it's thickness. I don't remember the details but remember the precision was surprising for such a simple apparatus. And by varying the length of the lever arms you can increase or decrease the precision.

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I think I agree too. Keep with your own personal same micrometer. Check the release torque (i will keep the same flat faced compression spring in a bag in the micrometer case and check the separation of faces is the same each time of thin wire measurement), clean faces and check zero before use . Is probably just as valid without going to full lab precision. And be aware the variable thickness of enamel has to be taken into account.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Oh-oh. I thought the insulation would be a much smaller percentage. It would also be difficult to determine the insulation type without a wire sample. It might be possible to disolve off the enamel (more probably silicon varnish) insulation and measure the remaining copper wire diameter. However, that puts us back to using either a micrometer or an optical reticule for measurement.

Agreed, if you don't mind a few additional sources of error. The above data sheet has some interesting footnotes: 6 - 44 AWG magnet wire will be furnished to dimensional standard with resistance values as a guideline. 45 - 55 AWG magnet wire will be furnished to resistance with the dimensions as a guideline. For #42awg, the diameter is the manufacturing control, with the resistance being whatever comes out. Although resistance tolerances are not specified, I suspect they will be substantial. With the difference in resistance between #42awg and #41awg at about 5% (and varying with temperature), methinks the resistance method is going to have some problems, unless you have a handy reference roll of wire handy with a known wire guage.

Good luck. You must have a really good micrometer with the requisite crystal ball accessory.

For single build wire, comparing #41 with #42awg: min max #41 .0029 .0033 #42 .0026 .0030 Note the overlap between .0029 to .0030. If your measurements land in this range, you have no way to determine if it's #41 or #42.

However, that assumes that you know that it's single build insulation. If I compare the diameters between single build and triple build: single triple min max #41 .0029 .0039 #42 .0026 .0035 we have an even larger overlap between .0029 and .0035. Any measurement landing in this range requires a decision from the crystal ball.

Also, based on my limited experience, measuring anything with a micrometer tends to be rather subjective. A friend that own a local machine shop once mumbled that he could pass a micrometer around the shop, ask everyone to make a specific measurment, and get a rather wide range of numbers depending upon technique. It's especially bad measuring soft items, such as plastic and possibly thin copper. Consistency of measurement is probably equally subjective. For anything in ten-thousanths his 12" Nikon optical comparator is the correct measuring instrument:

Unfortunately, my previous suggestion, of using an optical reticule to measure the diameter, will not work. It can probably resolve dimensions to within a thousanth, but to distinguish between two adjacent small wire diameters requires resolution down to the ten-thousanths. A hand loop and reticule are just not accurate enough. You might want to have the local machine shop, which presumably has an optical comparator, do the measuring.

The old guy that taught me insisted that the clutch on a mic is really meant as a safety release. If it clicks, you've already overtorqued it for precision measurement. That's why developing a "feel" for using it with a bit more finesse is important, especially in the sizes we're discussing here.

There's quite a selection of insulations available.

We use polyurethane for guitar pickups. We used to make coils for magnetic bearings, out of much thicker wire, and we used the heat-bondable stuff. An hour in the oven gives a nice solid coil without need for a vacuum bath. We had to do it that way because the coils were bobbinless. After winding, they went in the oven, then the four-piece winding bobbin was dismantled leaving the coil intact.

True enough. In my case, I'm trusting the label on the spool and merely verifying that I rec'd "min to nom" as requested. Not quite the same as reverse engineering an unknown pickup.

OTOH, I knew Seymour Duncan 30 years ago. Never wound pickups for him but did and still do some of his machining. He started with a couple of hundred square feet of loft and did the reverse engineering himself, counting off turns, measuring resistance, checking turns per layer, and otherwise analyzing the pickups he reproduced. So it's certainly possible.

Agreed, absolutely. Many people don't realize how much skill and experience is involved in something that looks so easy. My old boss used to spend half his time visiting customers' QC departments and teaching them how to use measuring instruments appropriately.

We are a local machine shop, but I never thought a comparator would be accurate enough for this application. I'm going to give it a try, though.

Smitty Two wrote in news:prestwhich- snipped-for-privacy@newsfarm.iad.highwinds-media.com:

It's called "precision" in Metrology.

precision: closeness of agreement between independent results of measurement obtained under stipulated conditions (»6) NOTES

1. Precision is a qualitative concept. Its quantitative counterpart is imprecision, which is computed as a standard deviation or a coefficient of variation of the measurement results.
2. Imprecision depends critically on the specified conditions.
3. Standard deviation expressing imprecision may depend on the value of the measurand; the phenomenon is called heteroscedasticity.

"repeatibility" is another factor

repeatability (of results of measurements): closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement (1) NOTES

1. Repeatability is a qualitative concept. Its quantitative counterpart is standard deviation of repeatability or coefficient of variation of repeatability of the measurement results.
2. Repeatability may depend on the value of the measurand.

I went thru USAF's Precision Measurement Equipment (PME) school back in

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But that realm of low forces is where grime/gummy lubricant etc affects the feel. I would say that , on my Moore & Wright one anyway, someone has determined that 2Kg clutch release force is about right for consistency. Obviously fine copper wire is going to compress but it will compress consistently, gauge for gauge, on a consistent 2 Kg force over the diameter of the plattens. So I'd disagree with your expert.

"N_Cook" wrote in news:h5gku8$s3t$ snipped-for-privacy@news.eternal-september.org:

you should not be making the mesaurement if the tool or workpiece is dirty.

Poor graduate.

Well, he's dead now, so he can't argue the point with you ... but I'd say if your mic is grimy and gummy, it's not a precision instrument now, even if it once was.

Sometimes, when a measurement is difficult, it's time to reformulate the problem.

If your objective is to rewind a coil, take your best shot at wire size and rewind the darn coil. If the resistance and inductance come out right, Isn't that what you really want? If not, try different wire. Self-resonant frequency will give you some idea whether your winding technique matches the original for capacitance between layers.

I do so agree, like minds and all that