English: The first radio transmitter, the spark oscillator invented by Heinrich Hertz in 1886, with which he demonstrated the existence of radio waves. It consisted of a dipole antenna made of two horizontal wires with metal plates for capacitance on the ends, with a spark gap between the inner ends, with the two sides connected to an induction coil. The induction coil applied pulses of high voltage to it. The coil charged the two sides of the antenna with opposite charges. When the antenna discharged through a spark across the spark gap, it caused standing waves of voltage in the antenna, which radiated the energy as radio waves. The frequency of the waves was determined by the length of the antenna, which functioned as a half-wave dipole. The short antennas used by Hertz generated high frequencies of 50 to 450 MHz. Using his oscillator, Marconi demonstrated standing waves, refraction, diffraction, and polarization of radio waves, establishing that both radio waves and light were electromagnetic waves that had been predicted by James Clerk Maxwell in 1864.