English: A Hertzian spark gap transmitter used in 1902 by Archie Frederick Collins (in picture) to investigate brain response to radio waves. This was the earliest type of radio transmitter, very similar to the ones Heinrich Hertz used in 1887 to discover radio waves. It consists of a half wave dipole antenna made of two horizontal wires ending in metal plates (E) with a spark gap (D) between them. When the operator presses the telegraph key switch (C), current from the automotive type lead-acid battery (B, bottom) powers the induction coil transformer (A) which produces pulses of high voltage which are applied to the spark gap. Sparks across the spark gap excite standing waves of current in the antenna, which radiates radio waves.

The dimensions of the antenna, acting as a half-wave dipole, determine the frequency of the radio waves generated. Since it is roughly one meter long, the radio waves have a wavelength of about 2 meters, resulting in a frequency of about 150 MHz; about the frequency used today by FM radio stations.