Using a phase-contrast microscope equipped with a high speed camera connected to a computer, as "demon", the principle has been actually demonstrated. In this experiment, information to energy conversion is performed on a Brownian particle by means of "feedback control"; that is, synchronizing the work given to the particle with the information obtained on its position. Computing energy balances for different feedback protocols, has confirmed that the Jarzynski equality requires a generalization that accounts for the amount of information involved in the feedback.