Kipling was not familiar with amplitude modulated transmission equipment, eg AM long wave or medium wave radio, nor with the demodulation of AM signals, at the time in which the story is placed. [The story was placed in 2065, but first published in 1912; Ed,] The spark transmitter was universal on board ship and for land based Wireless Telegraphy (WT).
A voltage of several thousand Volts at a current of 10 or 20 Amps was sparked across an air gap , this current was interrupted to produce Morse Code type dots and dashes…The pure beep of heterodyne reception had not yet arrived.
I remember a poem line of RK`s: ‘With crackling answer flung across the sky…’, referring to just such transmitter experience. The BBC or Science Museum Archives may have recordings of received signals.
The pioneering transatlantic Marconi transmitter at Clifden, Ireland, had a peat powered steam turbine generated system, the spark was found to be so powerful that an intermittent high pressure pulsed air blast was required to quench it. Large hangar structures held suspended capacitor elements, the damp climate must have caused some spectacular flash-overs… The phone cables were cut by insurgents and transmissions ceased- if ever an opportunity to talk revolution to the world, that was it- lost for want of trained operators.
When Lindbergh made his Irish landfall, he spotted the huge structures in the barren landscape, crash landed nearby and was brought in by the little turf collecting locomotive.
Bearing in mind the snarling sound emitted by a garage welder, burning 50 Volts at 60 Amps, the effect of being at the business end of a primitive Spark transmitter must have been a memorable one.
Various manufacturers had patented ways of quenching the spark, leading to distinctive tonal sounds and receiver operators could identify the transmitter thereby. For instance, the Eiffel Tower station had a distinctive purring note.(being French, of course.)
©John Peacocke 2008 All rights reserved