Before revision: Usually, a voltage amplifier with gain of serves as such a linear network, but also other devices can play this role: a man and a potentiometer in a potentiometric null-balance meter, an electromechanical integrator (servomechanisms using potentiometric feedback sensors), etc.
After revision: Usually, a voltage amplifier with gain of serves as such a linear network, but also other devices can play this role: a person and a potentiometer in a potentiometric null-balance meter, an electromechanical integrator (servomechanisms using potentiometric feedback sensors), etc.