Invention

Part 7 - Radio

https://www.youtube.com/watch?v=UN37QEmW_ns Hertz

https://www.youtube.com/watch?v=wOtAFgDSVGg Short demo of radio

https://www.youtube.com/watch?v=QZXYFr5YHew 6 min Herz reconstructed

Just as the wave theory seemed to triumph in 1905, Einstein explained the photoelectric effect.

Light had to be delivered as particles (photons) at sufficiently high frequency (having sufficient energy) in order to knock electrons out of metals. (However, it is still a mystery how light can behave both as waves and as particles).

Maxwell had proposed that light was simply short wavelength, electromagnetic waves, but no one was able to generate or detect electromagnetic waves of other wavelengths until, in 1887, a German physicist, Heinrich Rudolf Hertz, conclusively proved the existence of electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism. The unit of frequency, one cycle per second, was named the "Hertz" in his honour. 

 In 1886 Hertz, experimenting with a pair of Riess spirals (a pair of helically wound conductors placed one above the other to form an induction coil), noticed that discharging a Leyden jar into one of the coils would produce a spark across the terminals of the other coil.

This suggested the idea for a radio transmitter. He used a Ruhmkorff coil and two, one-metre long wires with a 7.5 mm spark gap between them ending in 30 cm zinc spheres at the ends for circuit resonance adjustments. The high voltage between the two wires created standing waves of very high frequency in the wires (about 50 MHz, similar to the frequency used in modern television transmitters) which radiated radio waves. The receiver was a simple half-wave dipole antenna with a micrometer spark gap. This apparatus radiated transverse waves from the wires to a zinc reflecting plate, about 12 metres from the oscillator, producing standing waves 4 metres long. 

Hertz used a ring detector to record how the wave's magnitude and direction varied. He also proved that Maxwell's waves travelled at the speed of light and that both the waves and light were forms of electromagnetic radiation obeying Maxwell's equations.

Hertz also demonstrated the polarization of radio waves by showing that signal strength was at a maximum when the antennas were parallel but zero when they were perpendicular to each other. He demonstrated refraction by bending radio waves through a prism made of pitch.

Unaware of the importance of his work, Hertz, replying to a question about applications, said, "It's of no use whatsoever . . . this is just an experiment that proves Maestro Maxwell was right. We just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there."

A Ruhmkorff, or spark, coil consisted of two coils of insulated wire wound around a common iron core forming an electrical transformer. The primary winding had tens or hundreds of turns of coarse wire. The secondary winding had up to a million turns of fine wire. A low-voltage direct current supply across the primary coil, repeatedly interrupted by a vibrating mechanical contact (called an interrupter), induced high voltage pulses across the secondary terminals through electromagnetic induction. Because of the large number of turns in the secondary coil, the secondary voltage pulse was typically many thousands of volts, sufficient to cause an electric spark to jump across an air gap separating the secondary coils terminals. 

(The spark coil was used for ignition coils in internal combustion engines, spark-gap radio transmitters, arc lighting and x-ray machines from the 1880s to the 1920s).

Spark-gap transmitters were the first type of radio transmitter, used from 1887 to about 1918. But, because they generated brief transient pulses of radio waves, they could not produce the continuous waves needed to carry audio (sound) in modern AM (audio modulation) or FM (frequency modulation) radio transmissions. Instead, they were used for radiotelegraphy where the operator switched the transmitter on and off with a telegraph key, creating pulses of radio waves to spell out text messages in Morse code.

About 1896, Guglielmo Marconi built the first practical spark gap transmitters and receivers for radiotelegraphy. They were widely used on ships, to communicate with shore and broadcast distress calls. They proved crucial in maritime rescues such as the 1912 Titanic disaster.

After 1918, transmitters, using vacuum tubes (valves) capable of producing continuous waves with greater range and less interference, were introduced. They could transmit a continuous audio signal which made spark transmitters obsolete by 1920. 

The radio signals produced by spark-gap transmitters had a wide bandwidth and were electrically "noisy" thereby creating radio frequency interference (RFI) with other radio transmissions. (This type of radio emission has been prohibited by international law since 1934, incidentally requiring all automobile spark ignition systems to be fitted with radio emission suppressors).

Oliver Lodge, Ferdinand Braun and other researchers were also experimenting with "Hertzian waves" (The term "radio waves" was not common until about 1910). With the invention of radio transmitters capable of emitting a continuous signal, radio broadcasting was soon introduced followed later by television. In 1909, Braun and Marconi received the Nobel Prize in physics for their "contributions to the development of wireless telegraphy".