The paper by Blandford and Znajek explores the possibility of extracting energy and angular momentum from a rotating Kerr black hole through electromagnetic processes. They demonstrate that when a rotating black hole is threaded by a strong magnetic field supported by external currents in an equatorial disc, an electric potential difference is induced. If the field strength is sufficiently large, the vacuum becomes unstable, leading to the production of electron-positron pairs and the formation of a force-free magnetosphere. This mechanism allows for the extraction of energy and angular momentum electromagnetically. The authors derive the fundamental equations governing stationary axisymmetric force-free electromagnetic fields in Kerr spacetime and show that charge cannot significantly contribute to the geometry of a rotating black hole. They also develop a perturbation technique to solve these equations for slowly rotating black holes and provide examples of solutions. The paper concludes with a discussion of the application of these ideas to active galactic nuclei, where the presence of a massive black hole surrounded by a magnetized accretion disc can explain the observed properties of these objects.The paper by Blandford and Znajek explores the possibility of extracting energy and angular momentum from a rotating Kerr black hole through electromagnetic processes. They demonstrate that when a rotating black hole is threaded by a strong magnetic field supported by external currents in an equatorial disc, an electric potential difference is induced. If the field strength is sufficiently large, the vacuum becomes unstable, leading to the production of electron-positron pairs and the formation of a force-free magnetosphere. This mechanism allows for the extraction of energy and angular momentum electromagnetically. The authors derive the fundamental equations governing stationary axisymmetric force-free electromagnetic fields in Kerr spacetime and show that charge cannot significantly contribute to the geometry of a rotating black hole. They also develop a perturbation technique to solve these equations for slowly rotating black holes and provide examples of solutions. The paper concludes with a discussion of the application of these ideas to active galactic nuclei, where the presence of a massive black hole surrounded by a magnetized accretion disc can explain the observed properties of these objects.