From Richard Stanley:
The parameter that you see most widely quoted (resistance in ohms) is, by itself pretty meaningless. The most important parameter by itself is inductance specified in henries. Resistance is of course measured with a common ohmmeter or multi-meter and is quoted as Direct Current (D.C.) resistance - inductance is measured with an impedance bridge. In the process of "balancing" the impedance bridge another parameter is quantified: Q or Q-factor. Any one of these by itself doesn't tell you much, but all three together do go a long way towards predicting a pickup's characteristics. These three parameters apply to any coil, but in a magnet-coil pickup, taken together they offer clues as to what the voicing (bass/treble or eq) of the output will be.
Copper magnet wire used in pickups is most often 42 gauge (#42) and has a resistance of about 1.70 ohms per foot. Typical pickups have about 5,000 feet of wire in them, hence a D.C. resistance of 8,500 ohms (8.5k ohms) is common. Copper has a significant thermal coefficient hence resistance is specified at a standard temperature of 68 degrees Fahrenheit/20 degree Celsius. The resistance reading of a pickup can vary considerably on the bench with handling. To get an accurate reading the pickup must stabilized at the test temperature.
Inductance varies as the square of the number of turns in the coil and its configuration. Typical inductance readings for two of the most common pickup types are: Fender single-coil types, 2.80 henries; Gibson-style humbucker types, 4.70 henries.
The inductance of pickups is also affected by peripheral and iron-core effects which typically contribute 40-50% of the total inductance. Iron core effects vary with magnet type and related pole structure design and are produced by eddy currents in the magnets and their fields, and pole structure. An interesting effect in practical application is the difference in voicing in pickups between those having alnico-type magnets and those with ceramic or Indox-type magnets, but which are otherwise identical. Since the ceramic/Indox-types are not electrically conductive they do not add to overall inductance. Higher inductance implies a voicing tilted toward lower EQ or bass. A pickup with lower inductance will generally sound brighter.
Another important parameter is distributed capacity which is the main culprit in limiting high frequency (treble) response. It is an expression of the capacitance among all of the turns of wire in the pickup and appears as a capacitance across (shunting) the output in the same relationship as the capacitor in a common tone control. Since capacitance is affected by surface area, proximity and difference in electrical potential it can be difficult to get a grip on this. Distributed capacity varies with number of turns of wire in the coil, insulation thickness and layup style. All else being equal a coil layed up in skew pattern will have lower distributed capacity (and greater high-end (treble) response than one where the individual turns are closer together.
The remaining parameter, Q-factor is an expression of how "pure" the inductance is. Resistance, inductance and distributed capacity are characteristics of all coils and contribute to the impedance of a coil and shape its EQ. Q or Q-factor is stated on a scale of 1-10. A Q of 10 would indicated a coil of nearly pure inductance with little dc resistance or distributed capacity. A Q on the low end indicates a high proportion of dc resistance and distributed capacity. A coil with high Q will have a closely focused transmission characteristic while a coil of lower Q will have a flatter transmission characteristic. Common pickups have a Q of about 2.0 and hence their transmission characteristic is quite broad.