1. Magnetic Field: A pickup coil relies on a changing magnetic field. This field can be generated in several ways, depending on the application:
* Permanent Magnet: The simplest case involves a permanent magnet moving relative to the coil. The magnet's field passes through the coil.
* Electromagnet: An electromagnet (coil with current flowing through it) can also generate the changing magnetic field. The strength of the electromagnet's field can be altered by changing the current, creating the necessary change in magnetic flux.
* Moving Ferromagnetic Material: A ferromagnetic material (like iron) moving near the coil, especially if it's magnetized, can alter the magnetic field through the coil.
2. Coil of Wire: The pickup coil itself is composed of many loops of wire. The more loops, the stronger the induced voltage.
3. Changing Magnetic Flux: The key is that the *magnetic flux* through the coil must change. Magnetic flux is the measure of the magnetic field passing through a given area (in this case, the area enclosed by the coil). This change can be caused by:
* Movement: The magnet or ferromagnetic material moving closer to or farther from the coil.
* Change in Field Strength: The strength of the magnetic field changing over time (e.g., an electromagnet's current fluctuating).
* Change in Coil Orientation: Altering the angle between the coil and the magnetic field lines also changes the flux.
4. Induced Voltage: According to Faraday's Law, a changing magnetic flux through a coil induces a voltage (electromotive force or EMF) across the coil's terminals. The magnitude of the induced voltage is proportional to:
* Rate of change of magnetic flux: Faster changes in the magnetic field lead to a larger induced voltage.
* Number of turns in the coil: More turns mean a larger induced voltage.
5. Output Signal: The induced voltage is the output signal of the pickup coil. This signal is often a weak AC signal reflecting the variations in the magnetic field. This signal is then often amplified and processed to be useful.
Examples of Pickup Coil Applications:
* Electric Guitars: The strings, when plucked, vibrate near a permanent magnet and coil, inducing a voltage that's amplified to create the sound.
* Metal Detectors: A coil generates a magnetic field, and changes in the field due to the presence of metal objects are detected by a second coil acting as a pickup coil.
* Crankshaft Position Sensors (in cars): A toothed wheel rotates near a coil, generating pulses as each tooth passes, providing information about engine speed and position.
* Tape Heads (in cassette players): The magnetic tape's fluctuating magnetization induces a voltage in the pickup coil, reproducing the recorded audio.
In essence, a pickup coil acts as a transducer, converting changes in a magnetic field into an electrical signal. The strength and frequency of the signal directly relate to the nature of the changing magnetic field.
