Saturation
current corresponds to the case when all the photoelectrons emitted by
the emitter plate C reach the collector plate A We now apply a negative (retarding)
potential to the plate A with respect to the
plate C and make it increasingly negative
gradually When the polarity is reversed,
the electrons are repelled and only the
sufficiently energetic electrons are able to
reach the collector A The photocurrent
is found to decrease rapidly until it drops
to zero at a certain sharply defined,
critical value of the negative potential V0
on the plate A