English: Diagram by Heinrich Hertz of his spark oscillator, the first radio transmitter, with which he discovered radio waves, from an 1888 paper. Hertz did not include many drawings of his equipment in his papers, and this drawing is cropped from a more elaborate drawing. The oscillator consists of a dipole antenna comprised of two collinear metal rods with capacitive metal plates (A, A') on the ends with a spark gap between them. The two sides of the antenna are connected to an induction coil (Rhumkorff coil) (J) powered by a battery (not shown). When Hertz pushed a switch in the primary circuit of the coil it would break the current in the primary, inducing a pulse of high voltage in the secondary which would charge the two sides of the antenna with electric charge. The charge would be discharged through a spark across the spark gap, causing brief oscillating standing waves of current in the antenna. This energy would be radiated by the antenna as a brief pulse of radio waves. Hertz detected the radio waves by tiny sparks across micrometer spark gaps between the ends of loops of wire (not shown) that acted as resonant receiving antennas tuned to the frequency of the transmitter.