This book uses a number of electro-thermal processes as an example to show that it is possible to develop new methods of monitoring and controlling electro-thermal processes using the specifics of the conversion of electrical energy in electric arc furnaces. It is used of the physical and chemical phenomena that accompany the passage of current and the conversion of electrical energy into thermal and chemical energy in the reaction zone for assessing the progress of the production process. The fact is that the materials in the reaction zone are in different states of matter, i.e. solid (the charge), liquid (the melt), and gaseous. The passage of the current in each of these cases has its own peculiarities which manifest themselves in the distortion of current and voltage curves.