The resistance that a material offers to electron flow depends on its properties, length and thickness. The more thinner and lengthy is a conductor, higher it will be the electric resistance associated.
  Although using a pencil mine (lead) thicker than copper string, in this experiment it is verified that the mine offers higher resistance than copper to the flow of electrons. This because the lead has an atomic structure (carbon) that doesn't allow a free movement of electrons (lower conductivity). In this experiment it was verified equally that as the lead length increases, the intensity of the light decreases. This because the electric resistance is increasing progressively.
  Finally, the pencil mine works as a reostate because it allows us to change the resistance of the electric system. Try it and learn science at home!