This can be a pickup that uses a DC power supply to charge one coil and make an electro magnet and the second coil is connected to an amplifier for picking up the string vibration. These pickups can be noisy and often the coils get hot and the pickup starts to change in tone. As the pickup (coil) gets warmer, the frequency decreases. The pickup can get bulky and add extra weight to the instrument. I’ve experimented using a DC power supply and electro magnets with lots of fun. As you increase the current in the coil, the magnet can become really magnetized and using a variable DC power supply, you can come up with some interesting sounds.