The very first radios were "sparkgap radios" (e.g. Marconi's
radio). They could only be used for radio telegraphy (i.e. sending
telegrams).
The transmitter was a high voltage sparkgap connected to a long
antenna wire. A message was sent by applying a high voltage across
the sparkgap each time the operator pressed the telegraph key,
generating a spark and causing the antenna wire to radiate very
wideband electromagnetic noise.
The receiver was a low voltage sparkgap connected to a long
antenna wire. When the antenna picked up any electromagnetic noise
the sparkgap would spark while that noise was being picked up.
However it was not practical to look for this spark, so a device
called a "coherer" was used to detect the spark and generate a
signal to drive a conventional telegraph sounder. The coherer was a
glass tube full of iron filings with an electrode at each end
touching the filings, usually the filings made poor contact and the
resistance of the coherer was high; but when the voltage across the
sparkgap became high enough to spark as the coherer was across that
sparkgap this voltage caused the filings to cohere and the
resistance of the coherer to became low, energizing the telegraph
sounder. Unfortunately the iron filings in the coherer stayed stuck
together causing the resistance to stay low, so a small
electromagnetically operated hammer tapped the coherer each time it
activated to knock the filings apart and reset it to its normal
high resistance state, deenergizing the telegraph sounder.
The biggest problem with such radios was the wideband
electromagnetic noise they used did not permit more than one
station to transmit at a time, if more than one station transmitted
at the same time the result was an unintelligible garble!
