Pickups are the heart (but far from the only factor) of the guitar’s sound. They affect guitar’s tonal character, output, and sensitivity (and in some cases, the ability to sound in tune). This column examines different pickups, their construction, and how differences in construction determine the way a pickup sounds.

The main variables in pickup construction which have an effect on sound are the number of coils, the shape of the coils, the amount of wire wound on the coil, magnet material, magnet placement, and the magnetic or magnetized area which senses the string’s vibrations. Seems like a lot of stuff, but his is really easy. There are, of course, other factors that affect the pickup’s sound such as coil wire diameter. Most vintage pickups were manufactured using 42 AWG enameled copper wire, although the use of 36 guage and 43 guage date back to some of the earliest pickups. Currently, pickups are manufactured from a variety a gauges ranging from 36 to 46. The most common reason to use thinner wire is that more will fit on a pickup bobbin. Note that some inexpensive pickups manufactured in other countries, while they may look like the real McCoy and may be on a guitar from a reputable manufacturer, may use non-conventional materials for pickup construction. These cheap pickups generally sound, well, cheap.

For the basics, we’ll start out with the single coil pickup. The same principals will apply to 2 coil (humbucking) pickups.

COIL SHAPE: The illustrations below show one tall coil (left) and one shorter, fatter coil (right).
RULE 1: Tall thin coils generally have a brighter sound than short fat ones.

COIL WINDINGS: The amount of windings on a coil will greatly affect the pickups tone and output.  More windings equals more power but at the cost of treble response.  Standard Fender Stratocaster pickups are wound with enough wire to register a resistance of 6K to 7k ohms.  A hot Strat pickup may be wound to 9K or more and will sound fatter and louder.  Both coils of a standard Gibson humbucker are wound under 4K ohms each for a total of under 8K ohms.  I've seen hot humbuckers wound up to twice that total or more.
RULE 2: The more windings, the louder the pickup will be and the less treble response it will have.

ALNICO vs. CERAMIC: Magnet material is usually one of two varieties: Alnico (aluminum, nickel, and cobalt) or ceramic.  Alnico was used in all popular vintage pickups and generally has a warm and smooth response.  Ceramic magnets tend to boost treble response and have become popular for pickups with extra coil winding to compensate for the treble loss.  Alnico magnets also lend to a smoother distortion tone with a prominent midrange while ceramic magnets can improve the clarity and high end grind of a distorted tone. There are many different formulas of Alnico and Ceramic magnets. In general, the stronger the magnet can be magnetized, The brighter it will soumd in a pickup.
RULE 3: Alnico magnets sound smooth and warm and distort smoothly, ceramic magnets sound brighter and distort cleaner but a bit less full.

POLE PIECES: Magnet placement is an important factor which is often overlooked in favor of sticking with conventional design.  Fender Strat pickups have 6 pole pieces which sense the string vibrations.  These 6 metal cylinders are magnets - no other magnets exist under the pickup.  Most other manufacturers use steel pole pieces (usually adjustable screws) and place one or more bar magnets under the pickup.  Here, the magnetic energy is transferred through the screws.  The amount of magnetic energy sensing the strings' vibrations is generally weaker than the magnetic energy of a Strat pickup.  The drawbacks of the Fender design include not being able to adjust the pole pieces and the phenomena of getting magnets too close to the strings which can lead to an out of tune sounding instrument (especially the low E string).  The advantage of the Fender design is increased sensitivity of the pickup.
RULE 4: Magnet pole pieces are not adjustable but increase power and sensitivity.  Steel pole pieces are adjustable and enable a variety of magnet structures to be placed below the coils, but are less sensitive to string vibrations.

MAGNETIC FIELD: The area along the length of the strings which is sensed by a pickup is another factor.  The perfect example of this are the blade-pole pickups offered by some manufacturers.  In these, a thin blade of steel senses the string vibrations at a small length of the string.  This results in an accurate but thin sounding pickup with emphasis on treble.  Pickups which use fat pole pieces or a fat bar pole piece will have more midrange emphasis and will be much more sensitive and powerful.
RULE 5: The more length of the string which a pickup is sensing, the more midrange, sensitivity, and power it will have.

MAGNET STRENGTH: Relative to pole piece construction, magnetic field, and type of magnet is the strength of the magnetic field. This also relates to the proximity of the pickup to the strings. The stronger the magnetic field sensing the strings vibration, the more treble response you will hear.. 
RULE 6: The stronger the magnetic field sensing the strings, the more treble you will hear from the pickup.

HUMBUCKING: This bring us to the 2 coil humbucker.  Here, a second coil is added with coil windings in the opposite direction and opposite magnetic polarity.  By doing this, the second coil cancels out the noise created by the first coil.  But, we also have widened the length of string sensed by the pickup, giving us more midrange.  Also, any frequencies sensed by both coils will also be canceled out (ok, it's a little more complicated than that).  Basically, since treble frequencies have a very short wavelength, there is a greater chance that both coils will "see" these frequencies the same and will be canceled out.  Therefore, humbucking pickups will always cancel out some treble response -- uh, with the exception of air coils and stacked humbuckers.  Air coils are a second coil without any magnets (popularized by Alembic).  Stacked humbuckers are similar to air coils in that the second coil does less sensing of the string than the primary coil.  In both air coils and stacked humbuckers, the single coil sound can be preserved, but because the signal has to pass through extra wire, it loses a little bit of power.
RULE 7: Standard humbuckers cancel hum and some treble response.  Air coil and stacked humbuckers cancel hum and preserve the single coil tone at the cost of a bit of power.

I've recently renewed my liking for good humbucking pickups.  I've learned a few things from Jeff at HighOrder Pickups.  Jeff custom winds humbuckers to your desired tone and type of guitar to achieve the sound you're looking for.  Highly recommended.  I recently installed a pair of HighOrder Alnico 2 humbuckers wound to 8.5k (bridge) and 8.0k (neck) in my 1977 Ibanez LP.  Man, I am blown away.  I honestly have never heard a humbucker sound so good!  I've heard a lot of humbuckers but these HighOrder pickups sound better than any of them.  Absolutely the most balanced tone with sizzle and clarity, and artificial harmonics just jump out.  Switch the pickup to a single coil and it's the best, chimey, single coil sound I've heard from a 'bucker.  Now, I'm not trying to boost anyone's sales or trying to get any special endorsement deals - I just want to pass along a recommendation to anyone who has a humbucker-equipped guitar that they wish would sound better.  HighOrder pickups have made me love one of my favorite, old guitars again, and that's a great thing!!