English: The first spark gap oscillator built by German scientist Heinrich Hertz around 1886, the first radio transmitter, with which Hertz discovered radio waves. It consists of two 1 meter copper wires, supported on wax insulators, with a 7.5 mm spark gap between the inner ends, with 30 cm zinc balls on the outer ends. He used different sized balls to change the capacitance, in order to change the frequency. It functioned as a half wave dipole antenna. An induction coil (not shown) applied a high voltage of about 20,000 volts between the two sides, creating sparks across the gap between the small balls at center. The sparks caused oscillating standing waves of radio current in the antenna at its resonant frequency, radiating radio waves. The frequency of the waves was approximately 50 MHz, about the frequency of modern television broadcasting transmitters.

Caption: "The First Oscillator of Hertz. Two copper wires, each 1 metre in length, supported on rods of sealing wax. The large spheres are of sheet zinc, and are 30 centimetres in diameter. Base 260 x 7.5 centimetres"