For a phono cartridge to generate current and voltage, something must move: a coil of wire (as in a moving-coil cartridge), or a magnet (as in a moving-magnet type), or a tiny piece of iron (a moving-iron type). In those rare cartridges that depart from the electricity-generating principle of the ones described above, it can be a displacement-measuring device in which a moving shutter modulates a light source to vary a supplied voltage (as in an optical cartridge), or one in which voltage is modulated when a tiny chip of silicon crystal is squeezed by a moving element, which varies the chip's electrical resistance (as in a strain-gauge cartridge). But regardless of what it is that moves in a cartridge, something has to.