Electricians working with high voltage are like divers: they need to see what's happening deep underwater without going down themselves. What's needed is a sensor that stays away from the dangerous current and relays information to the surface. Usually, light is used: a beam passes through a transparent crystal, and the electric field slightly rotates it. But this method has a weak point: it's sensitive to stray fields and noise.
Engineers in Germany took a different approach. They used two materials: a piezoelectric, which compresses or stretches under voltage, and a magnetic film, which changes its magnetic properties when squeezed. They bonded them together. When voltage is applied to the sensor, the piezoelectric presses on the film, the tiny magnetic arrows inside the film rotate, and this changes how the film rotates the light beam passing through it. The angle of the beam's rotation is the signal.
This way, both direct and alternating voltages can be measured. The precision is millivolts per square root of hertz, meaning the sensor can hear very quiet electrical 'voices.' Unlike previous optical sensors, it's not afraid of strong electromagnetic interference and doesn't require direct contact with the circuit. It's like an interpreter sitting in a safe room, reading lips through glass.
Now power engineers have a tool that can monitor voltage in the harshest places: substations, electric trains, laboratories with powerful equipment. And all this from just a beam of light, which doesn't conduct electricity and doesn't create dangerous paths for current.
Piezoelectrics are also used in everyday life: in gas stove lighters, the spark is struck by a blow on a piezoelectric crystal.