Chapter 3 Electromagnetic Induction
Atomically, magnets can be conceived of as an electrical charge in motion. This charge creates a magnetic field around the magnet, which exerts a constant force on the magnetic atoms. The process can be thought of as a magnetic spectrum in which curved, serried lines of magnetic force or flux lines arise from a north pole and flow into a south pole. When soft magnetic metals are brought within range, they draw the flux lines into themselves and as a result, change the nature of the magnetic field. The newly created magnetic field may be considered a flux linkage between several combined fields that surround all the soft magnetic parts of the pickup as well as the strings above it. Hence, touched strings are actually vibrating magnets at some distance from the coil. The centrally located coil embraces a certain amount or flux and acts as a conductor, thus bringing a magnetic field into association with an electric current; this phenomenon is called electromagnetic induction.
Induction takes place when vibrating strings cause the flux linkage to alter rapidly, as a result of which an electro-motive-force (emf), or voltage, is induced in the coil. The magnitude of the induced emf is proportional to the number of turns in the coil, i.e, an equal voltage is induced in each separate coil winding. Since the windings are in series, the total output voltage as it reaches a Z load (see chapter 4) in the form of AC is thus a kind of addition sum. Yet, there are other factors involved, such as the rate at which the flux linkage alters, the extent to which the strings have been induced magnetically, and the self induction properties of the pickup.
Prototypes of pickups can best be designed with average specs in mind. Average specs are a logical starting point for modifications and guarantee a proper match to the input characteristics of musical instrument amplifiers. Most modern amps use a pre-gain control as part of an active pre-amp circuitry whose input dynamic range is calculated for output voltages varying from –40 dBV, 10 mV RMS to 0 dBV,1 Volt peak, as produced by medium and high Z pickups. Consequently, experiments can be performed without too tight restrictions, although employing pre-amps to boost the signal level of low Z pickups, or having the input characteristics of amps modified by a qualified technician really opens up possibilities. It is worth mentioning, in addition, that during induction, an emf is induced in all metal conductors present within the altering flux linkage. When induced in solid conductors such as covers, the emf causes a random flow of so-called eddy currents. The magnetic field produced by these eddy currents acts in opposition to the altering flux linkage and, as a result, cancels out high frequencies. This only happens, however, if a coil is shielded in any way by metallic thicker than about .003’’.
Alongside induction caused by string vibrations or a similar mechanical motion, there also exists the phenomenon of mutual induction. Mutual induction causes an emf to be induced in one coil when it is linked by an alternating flux to another coil. This phenomenon makes possible an alternative method to the complicated graphs and specs approach for testing a pickup’s sound potential: audible tape- recorded results. In contrast to the sophisticated equipment required for making various complicated measurements, all that you need here is an audio set. Proceed as follows: switch on your set, and an audio signal (AC) will be fed into the voice coil of each- preferably full range- loudspeaker.
This signal causes the voice coil to move in accordance with the change in the direction of the AC. The AC passing through the voice coil will create a surrounding flux pattern which will built and collapse with the same frequency as the impressed current. All you need now is to hold up your pickup at a distance of app.1’’ in front of the voice coil to bring about a mutual flux interchange that will induce an emf in the pickup; current flow can then be supplied to a low or high Z deck input. Once you have set your deck’s recording level control to match the pickup’s output level – depending on the strength of the current in the voice coil, the number of coil windings in the pickup and the distance between the voice coil and the pickup – you can record any piece of music directly. This simple testing method offers extensive possibilities – with the added advantage that it provides constant circumstances; and so, three cheers for applied physics.