The NPN sensors delivered standard with the HLCP product are "inductive" sensors, which by definition consist internally of an induction wire loop (or coil). Electric current applied to the sensor generates a magnetic field in the wire coil inside the sensor. As a metallic object (in this case, the piston rod inside the cylinder assembly) approaches the sensor, the "inductance" of the sensor's coil changes. In simple terms, the presence of the piston rod in proximity to the sensor changes or influences the magnetic field, which alters the signal across the sensor wires. Without power feed to the sensor, there is no magnetic field in the sensor's coil and no signal change is read.