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.