Two equal metal spheres _A_ and _B_, each twelve inches in diameter, and each carrying copper rods _C_ and _D_, are placed as indicated in the diagram given here. At _E_ is an opening of about three-tenths of an inch in length, the so-called _air-gap_. By means of two wires _e_ and _f_, connected to an electrical machine, the spheres are charged, one receiving a positive electrical charge denoted by the (+) sign, and the other a negative one, denoted by the (-) sign. The air-gap _E_ insulates one sphere from the other, and its function is to make it possible for the electrical machine to increase the two charges until a very high electrical tension is reached. When the electrical tension between the two charges, acting through the air-gap _E_, is sufficiently high, then the insulating power of the air-gap is overstrained and suddenly it breaks down and becomes conductive and permits the two charges to rush toward each other. The conductivity of the air-gap suspends the action of the charging machine. A large current passes then between the two spheres along the rods and through the air-gap _E_ which is heated by the current to white heat. It becomes then a very good conductor and permits the charges to pass through it easily. The collapse of the air-gap is reported by the sharp crack of the electrical spark which is due to the very sudden heating and expansion of the air in the air-gap produced by the passage of the electrical current. It is a miniature lightning. The two charges reunite, the spheres are discharged, and after that the air-gap _E_ recovers quickly from its breakdown and becomes an insulator again. The process is then repeated by the action of the machine and a rapid succession of sparks can be maintained, each one of them announcing by the crack of the spark the reunion of the charges that had been pulled apart and forced to the surfaces _A_ and _B_ by the action of the electrical generator. All this was known long before Hertz. The first experiment of this kind I saw in Panchevo in my boyhood days, when my Slovenian teacher Kos explained to me the theory of lightning according to the views of Benjamin Franklin, a theory which clashed with the St. Elijah legend of Idvor and nearly proved me guilty of heresy. But there was something in these electrical discharges that Benjamin Franklin did not know, and that knowledge was first suggested by another great American scientist, a scientist even greater than Benjamin Franklin was in his day.