  The ac Josephson effect phenomenon is studied in fractional quantum Hall
states (FQHS). Usual quantum Hall devices have an electron layer of uniform
thickness. We consider a new quantum Hall device. The device has a narrow part
that is vertical to the direction of the current. The narrow part has a smaller
electron layer thickness than in the other part. The ac Josephson effect might
be observed when the magnetic field or the electric current is modulated by an
oscillation with a constant frequency value f. Steps of the voltage appear when
an electric current exceed a threshold value. The step value V is related to
the transfer charge Q as V=(2 pi hbar f)/Q. We examine how the value of the
transfer charge Q depends on the fractional filling factor nu. The nu -
dependence of Q is affected by the wave function of the fractional quantum Hall
state. We classify the value of Q according to the wave function type.
Accordingly, observation of the ac Josephson effect in FQHS clarifies the
structure of the fractional quantum Hall states.
