# A coil I built has divergent measurements respective to wire resistance and AWG size

I'm not exactly into building precision coils, but I decided to make one for a subwoofer voice coil.

I used what was supposed to be AWG 28 wire, it completely fit the diameter. I wound it tightly over a cylindrical metal piece and it took 15 turns until the length of the coil was 5 mm.

So I went on and built a 25 mm tall, 40 mm diameter voice coil with two layers (orthocyclic winding) and it has 78 turns on each layer (completely compatible with AWG 28). The legs are 10 cm each.

But when I measured the resistance with my multimeter it read 5.8 Ω. I was expecting 4.2 Ω.

Noting that I'm already accounting for the resistance of the multimeter leads and the electrical connection with the legs, I measured the resistance between two points in each leg and they both read 0.1 Ω (the whole coil measured 6 Ω, hence 5.8 Ω actual resistance).

I also tested my multimeter with resistors I have. A 100 Ω resistor measured 101.1 Ω, a 22 resistor measured 22.1 Ω and a 4.7 Ω resistor measured 5.1 Ω (all of that without subtracting the resistance of the leads this time, and all of them being 5% tolerance resistors).

It seems the multimeter is reasonably calibrated, values from 100 to 4.7 Ω show very little discrepancy.

What could be happening here?

I don't see any damage that could suggest fractures along the wire.

Could this actually be AWG 29 wire with a thick insulation?

Or a cheap wire with poor copper content?

Ps.: I wound it in an aluminium former which I then removed from it. Now there's only the copper wire with a tiny bit of resin that probably doesn't amount to 0.2 grams. I weighed it on a scale and it read 11 grams. With the error it could be 10 or 12, but an AWG 28 wire with 19.8 meters of length and a cross-section area of 0.08 mm² should weigh a bit over 14 grams. I tested my scale with water and a syringe and it seems the errors are always inside +- 1 gram (as expected.) This somewhy fits better with the weight AWG 29 wire would have with the same total length of wire.

• How long was your wire - you haven't demonstrated that you know how much wire you used. Apr 24 '20 at 8:32
• 78 turns per layer. How many layers? Apr 24 '20 at 8:38
• @Transistor Two layers (like most voice coils). Seems like I accidentally deleted the word "two" from the text. This gives 19.8 meters of wire. Apr 24 '20 at 8:39
• Noting that even if I added 1 meter more and rounded it to 21 meters this still doesn't give anything near 5.8 ohms. Apr 24 '20 at 8:41
• I've confirmed your calculations, allowed for increased diameter on first and second layer and tails and get 4.268 $\Omega$. Apr 24 '20 at 8:44