Other systems work with a sinusoidally modulated light source instead of the pulse source. Thus the magnetic flux through the stator varies sinusoidally at a rapid rate. In a circuit with alternating current, the value of voltage and current vary sinusoidally. It is then sinusoidally vibrated, typically through the use of a piezoelectric material. A pendant droplet with surface active molecules in it is formed and pulsated sinusoidally. This angle varies sinusoidally through the year and is called B0.