English: Artist's conception of German scientist
Heinrich Hertz historic discovery of
radio waves in 1886. To generate the waves Hertz used a
spark gap radio transmitter (rear) consisting of a
spark gap between two brass balls attached to a
half-wave dipole antenna consisting of two wires with metal plates at the ends, powered by a
Ruhmkorff coil with primary current supplied by a set of liquid batteries on the lower table. The Ruhmkorff coil generated pulses of high voltage which caused sparks to jump between the brass balls. Each spark excited oscillating radio frequency currents in the antenna, which were radiated as
electromagnetic waves (radio waves). For a receiver, Hertz used a simple loop of wire
(in his hands) with a narrow gap between the ends, forming a narrow
spark gap (this picture is slightly inaccurate; Hertz actually used a micrometer gap consisting of an adjustable thumbscrew with its end close to the opposite electrode, to precisely measure the spark length). The length of the wire was a quarter wavelength, so the loop formed a resonant
loop antenna, and the radio waves excited a voltage in the wire. So each spark of the transmitter excited a spark in the receiver loop.
The transmitter shown was one common type Hertz used, but not necessarily the one with which he made his discovery. Hertz experimented with various types of antenna, from a bare spark gap to dipoles 8 feet long, with square metal plates, round metal plates, and metal balls on their ends. Also, the size of the receiver spark gap is exaggerated in the drawing. The gap in the receiving loop was very small, less than a millimeter, to detect the weak voltages induced. Some of his receiving loops used a gap adjusted by a micrometer.