Use wire wrapped around a permanent magnet to generate an electrical field in response to the vibration of a metal string. Magnetic pickups have a number of factors that effect the quality of sound generated by them.

Require metal strings on the instrument
Require Wire ideally from 40-45 AWG¹⁾ to work as passive pickups.
Require the wire to be isolated

The degree of intensity (Gauss) of a magnet greatly effects the kind of sound output the pickup will generate and the stronger the Gauss the more variety of frequencies that will be picked up by the magnet. Weaker magnets generally preference low frequencies over higher ones. The vast majority of pickups use alnico or ferrite magnets. Both of these magnets can be selectively magnetized using strong magnets to the desired Gauss. ²⁾When picking the intensity of a magnet considerations with the thickness and amount of copper wire should also be taken into account. The pairing of a magnet and wire can match the general frequency response or can compliment one another. For instance a strong magnet that allows a great deal of high frequencies can be paired with a thinner wire that prefers lower frequencies but comes with the advantage of generating a stronger signal strength. This pairing allows for some of the limitations of thin wire to be compensated.

There are many methods for wrapping the wire, with the primary methods being to uniformly wind the wire and to do something called scatter winding. ³⁾ The more wire that is wound the higher the capacitance and the more its favors low frequencies over higher frequencies of sound. Uniformly wound wire is able to have more rotations around a magnet and has a much greater tendency to favor low frequencies. Wire used for wrapping magnets is generally covered in a shielding material to prevent current from being exchanged and to limit oxidization of the wire. The thickness of this coating may also have an effect on the frequency response by adding material to the thickness of the wire. Thicker coatings allow less rotations of the wire around a magnet and have a minor effect on frequency response. Thinner coatings remove more high frequencies than thicker ones, in this way the coatings have a similar effect to wrapping methods as they determine how tightly packed the wire is against a magnet, with tighter packing reducing high frequency response.

The thinner a wire is (44 gauge is frequently quoted as ideal) the more signal strength the wire will generate ⁴⁾, but this comes at a compromise with the amount of frequencies that will be evenly picked up by the wire, with low frequencies being favored on thinner gauge wires. Thicker wire have a more even distribution of frequencies that it picks up but it is also far less capable of generating a strong signal strength. Commercial pickups limit their wire gauge to 42 gauge but in homemade pickups a gauge as low as 40 can still be used and still function properly as a passive pickup. With thicker wire than 40 gauge some kind of preamplification is needed to increase the signal strength, and are completely doable but require additional considerations when it comes to functionality. 44 gauge is also widely viewed as the commercial limit for reduction of high frequency response. Wire that is thinner than 44 gauge will have a lower response to high frequencies to the degree that is is commonly viewed as a determent to the pickup quality. An additional note is that the higher the gauge of the wire the more delicate the wire is, and 44 gauge is considered difficult to work with as it easily brakes if mishandled. Thinner wire have even less resistance to error and will brake even under controlled and careful winding. The copper wire that is wrapped around a pickup is wrapped thousands of time around the magnet. In the case of the 42 gauge copper wire the wire is generally wrapped 4000 times. ⁵⁾

After the pickup has been constructed the layers of copper wire and magnets need to be completely encased in a neutral material to prevent any kind of motion of the wires. This is generally doing by submerging them in hot paraffin wax of shellac. In many cases they are also placed under vacuum pressure to remove any air that remains.

Commonly called lipstick pickups.

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A low impedance pickup works in a similar way to a high impedance pickup but instead of using many loops of a thin copper wire around a magnet a thick loop of any kind of conductive metal such as copper or aluminum can be used that is then connected to a high impedance transformer (a step up or boost transformer) that than increases the signal strength for a amplifier. I simple form of this type of pickup can be built by making a C shape of metal wire or a cut metal plate that is wrapped around a bar magnet and connecting it to a transformer and than into an amplifier. ⁸⁾

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Example of a simple magnetic pickup with thicker than ideal wire ¹⁰⁾
Instructions on how to build a copper wire winding machine ^{11) 12)}
Making a pickup winder from a sewing machine and calculator ¹³⁾