In musical instruments, the guitar is still one of the most famous and said to be one of the coolest instruments used from years ago until today. There are various kinds of guitars, but here we will focus on the electric guitar - specifically on how it produces its sound when played. A transducer, called a “pickup” is placed across the strings of the electric guitar. This device is a passive device, meaning that there is no source supplying power for it to function. The pickup operates through what we call “Electromagnetic Induction”.
An electric guitar pickup is basically made of a copper wire coil, a bar magnet and pole pieces. With these components combined together, an electromagnetic device is created.
Reviewing the Faraday’s Law of Electromagnetic Induction, it states that moving a magnet “in '' and “out” of a copper wire coil, a current is induced into the coil by the physical movement of the magnetic flux inside it. Likewise, if we keep the magnet stationary and move the copper wire coil back and forth within a magnetic field, an electric current is also induced in the coil. Then, by either moving the wire or changing the magnetic field, we can induce a voltage and current within the coil. This process is known as Electromagnetic Induction and is the basic principle of operation of a guitar pickup.
Faraday’s Law tells us we need a changing magnetic field to make an electric current, induced on the coil, but on a guitar, how does the magnetic field from the static permanent magnets change? That’s where the string comes into play.
On an electric guitar, the strings are magnetized by the magnet in the pickups. This creates a magnetic field around the strings that is captured by the coils. Once a guitar player starts to strum or pluck on the strings, vibrations are created. In this scenario, the strings act as a magnet moving around the coil. As stated above in regards to Faraday’s Law of Induction, this moving string induces a signal in the pickup. The signal then goes out from your guitar to be amplified.
The strength of the signal depends on the number of coils of the pickup used, the gauge of the string used, and the magnitude of how the string is plucked. Typically, a signal produced by a guitar pickup is just a few hundred millivolts (mV). This induced signal is then sent to an amplifier for it to be amplified and heard.
Though all pickups are made with the same concept, they usually do not sound the same. This is referred to as the “tone” of the pickup. Technically, the tone of the guitar pickup is just the frequency response of the pickup. The frequency response is mainly affected by the number of turns of the coil, impedance, and coil configuration used in the pickup (single, dual, quad, hex).
The technology used in guitar pickups nowadays has not changed much from the first time it was released. Though many variants have already evolved from the original single coil pickups, variants such as hum cancelling pickups, hexcoil pickups, low-Z, full-range and active pickups, they are all just based on the original concept.