The professional community was surprised, and skeptical, when Marconi began achieving reliable communication over long distances. Here is what they deemed knowable at the time they began paying attention.
They wondered what it was that activated the coherer. Magnetism? That doesn’t work, they found. So it was something else. Whatever it is, it even works in vacuums. ‘Making and breaking’ current in an electromagnet, with a ‘vibrator’ or an ‘interrupter’, is how you made a transformer create whatever it was; or you could use alternating current. Artificial daylight apparatus activated coherers, they saw. So did trolley lines. Both of these generated small amounts of spark. So maybe it had to do with spark. But not everyone thought spark was strictly necessary. W.J. Clark told the New York American Institute of Electrical Engineers in 1898 that he was able to transmit short distances with no gap discharge at all. Marconi, he said, was surprised by this.
As for what these emanating currents did, Heaviside’s model was the gold standard for modelling them. (Institute president in 1898, Arthur Kennelly (pictured), was so fascinated by this that propagation experts three years later came to speak of a Heaviside-Kennelly layer in the atmosphere.) The emanating currents go in waves, they could tell for sure. Wave shape seemed to them like donuts. This seemed justified by J.J. Thomson following Maxwell. ‘Hertz waves’ and ‘Lodge waves’ looked like different things, however. Also, waves on the ground didn’t move like waves in the air, they could see as well. They reasoned that the surface of the earth ‘must be a conductor’. They could even tell that waves polarize, depending on whether antennas are vertical or horizontal. Transmitter builders were noticing that finer secondary coil wire worked better, too, though no one could say why.
Transmitting powers matter, they understood. Range goes up as power goes up. They reckoned that something like 95% of a spark coil’s power is used up in heat. They wondered if one could abate this simply with air-cooling, and make a better transmitter. Marconi’s school of thought in 1899 had it that you could also increase the length of your spark to increase your range. Longer spark length ought to mean stronger signals, owing to more EMF. But efficiency was a problem here. Each spark ‘is followed by a series of oscillations’, opined one Dr. Pupin (who also suspected that the number of sparks per second has something to do with the frequency of the signal). He imagined that the sparks dampened each other, depending on how rapidly they came. This attenuates signal strength (and it also generates spurious emissions on unknown and unknowable frequencies). He wished it were possible to generate oscillations without sparks. Hertz too had shown the importance at very high frequencies of finding some way to dampen rapidly decaying waves, for efficiency’s sake. People were talking about ‘decrement’, a special term for the decay rate of oscillations. Overcoming this problem was the rationale behind the ‘quenched gap’ a few years hence.
Marconi’s receiving apparatus was considered insensitive in 1899, a generally troublesome device worthy of abandonment as soon as possible. Coherers limited Morse code speed, to about 20 words per minute. No one had yet made exact measurements of what a coherer could do, partly because better techniques were already in sight. The super-sensitive receiver idea in 1898, the coherer being recognised as a blunt instrument, was a galvanometer. Whatever the receiver, they were inclined to think that signal strength that dissipated over distance requires a longer antenna to receive, because more wire accrues more volts.
Changing wire length brought its own problems. Engineers were aware of frequency changes, in a rudimentary way, and conceived of the possibility of tuning signals. 8-500 ‘breaks’ per second was the norm in 1898 spark transformers, and they could hear the corresponding variations in the musical pitch of the spark. This might be a way of tuning, they thought. Or maybe tuning was going to be a matter of condensers at the antenna feed-point, that control ‘damping’. They weren’t sure. C.O. Mailloux was adamant that transmitted waves resonate (and he used that word) at the receiving end only if the two sets were ‘synchronized’ with each other, so that the waves followed ‘in phase’. What this meant mathematically he wasn’t sure yet. Marconi himself was not even clear what the relationship was supposed to be between the height of sending and receiving wires. But broadly speaking, professionals knew that the electrical properties of transmitting and receiving wires ought to be as congruent as possible. They also knew that antenna size related to wavelength, and logically, that frequency is a function of wavelength. Fessenden declared that wave-length ‘is about four times the length of the wire’ used for transmitting. Thus it is that late Victorians were probably using quarter-wave verticals worked against the ground. (They could also tell that antenna ground connections were better on wet days.)
Says who: ‘The Possibilities of Wireless Telegraphy.’ Transactions of the American Institute of Electrical Engineers. 1899. 607-628. This is the transcript of a New York meeting, held on the same night as a parallel Institute meeting, in Chicago, to discuss Marconi, his assumptions, and his methods.
Photo: IEEE Global History Network, link here.
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