How to measure the diameter of an electrical wire I'm working on a project and I need to measure the diameter of a thin bare copper wire but I don't know how. Can you please tell me a way to measure the diameter of a wire.
Is there a way to measure it using diffraction?
 A: Laser pointer, wire, screen at known distance.
You will see the following diffraction pattern:

(not trying to make the best image... exposure could have been better, and I could have put a beam stop in in order to avoid the overexposure of the central beam.)
The point is that I can see a series of "blobs" that correspond to diffraction peaks from light that goes around my wire. I count slightly less than 10 peaks for 3 squares on my paper (1/4" squares), with a green laser pointer (wavelength 532 nm) at a distance of about 2.5 m from the screen.
From this you can calculate the thickness of the wire. By increasing the distance, and in particular by increasing the distance until you get an integer number of peaks falling on an integer number of squares on your grid, you can get almost any accuracy you want with things you already have lying around. And if you can't bend your wire, or you don't have enough to stack it up, that doesn't matter either.
The equation for the peak spacing is given from basic straight slit diffraction: for wire thickness $d$, distance to the screen $D$, wavelength $\lambda$, the spacing $w$ between peaks is given by
$$w = \frac{\lambda D}{d}$$
And so the thickness of the wire can be deduced from
$$d = \frac{\lambda D}{w}$$
In the above case, I calculate that $d = 0.69 mm$ diameter - indeed, it was a pretty thin wire I had lying around (somewhat smaller than 1 mm).
You can get quite good accuracy with this method, assuming that you have a known wavelength for your laser pointer. For greater control over the measurement, you could rotate the graph paper until you exactly found an integer number of blobs between your lines; the angle would give you some "fine tuning" of the measurement. And you can see quite easily see all the way out to the 20th peak; if we assume you can center these peaks better than 1/4 of their spacing (really that is not hard) your accuracy will be better than 2% ($\frac{\sqrt{2}}{4 \cdot 20}$). With a bit of care you can do even better. With stuff you have lying around your office.
Postscript
I just repeated this slightly more carefully with a wire I had at home. Measured diameter was 0.46 - 0.49 mm (often wire is not perfectly circular... As this measurement just confirmed). I set the pointer and wire up much further from the wall (28.5 1 foot tiles - right across the big room) and could see 26 fringes (you lose count near the middle but can extrapolate by counting what you can: I see 9 peaks on 14 squares, and N peaks on just over 40. I conclude N=26). 
I determine the wire to be 0.47 mm diameter from this measurement - right where the calipers put it...

A: I don't know what accuracy you need, but you can wind (densely), say, 100 turns of wire on a cylinder and measure the length of the coil.
EDIT: another approach (which can be more accurate, if you know what material you have): take a long piece of wire and weigh it. There are some ways to measure density as well.
A: You can first measure the length of the wire, then put the wire into the water, and see the volume change of the water. Then use $\pi r^2 = V/L$ to get the diameter. 
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*If you need more accuracy, maybe you can either coil the wire on a pencil or some other object many times, then you can measure the the length of many diameters, like in this graph. Then the error could be decrease.

*Or you can investigate about the interference pattern about your wire, this is just a proposal, here is a link about diffraction of hair and wire, see if it helps(it should be useful if your wire is very thin and it should provide a good precision).
A: You can also measure the electric resistance of a long piece of the wire and the calculate the cross section of the wire using the fact that the specific resistivity of copper (at 20 C) is $1.68\times 10^{-8}$ Ohm m.
A: Using optics, you say? Well, you make a Michelson interferometer, with the movable mirror referenced to a mechanical stop. You move the mirror away from the stop, insert the wire and close the gap until the wire is held lightly between the base of the mirror and the stop. Now pull out the wire and slowly close the gap until the mirror makes contact with the base, counting fringes as you do so. 
Alternatively, you can make a small aperture and lay parallel wires across the aperture with the wires touching each other. Remove every other wire and you have made a diffraction grating. Use a laser pointer as a light source and measure the diffraction angle. This can be used to calculate the wire spacing and therefore the wire size.
A: Technically Micrometers or more commonly called as Screw  Gauge are used to calculate diameter or radius of thin wires in physics labs you can refer to this article:
https://en.wikipedia.org/?title=Micrometer
A: A micrometer would be the preferred method.  A caliper would not be appropriate because it would not give you enough precision.  
A: copper wires can be measured by using screw gauge. read your text for how to use screw gauge. and use moving microscope to measure the same by optically
