This is the 'Joule thief'- a simple circuit that allows you to run a led light with a 'dead' battery.
This battery measures 0.6 volts and would normally be considered dead.The red led light needs at least 1.8 Volts to light up.Can this battery run this light?Of course not.Unless you use a joule thief!The thief brings up the energy from the dead battery to a level high enough to run the light.
See for yourself.
ferrite coil .I got mine from a burnt cfl (fluorescent) lamp.This one has about 10 turns of wire.You can experiment with more or less turns and see what happens.
bc547b transistor (many others might work)