I recently saw the PBS program Rescue at Sea, and highly recommend it as a fascinating and accurate account of an early episode in wireless telegraphy. The event was the collision off Nantucket of the SS Florida, inbound, and the SS Republic, outbound on a foggy night in the winter of 1908. The wireless brought the SS Baltic to the rescue. The only fatalities were in the collision itself; over 1500 were rescued, with no one lost at sea. The Republic's Marconi operator, Jack Binns, became famous for a period, and used his popular fame, unsuccessfully, to urge that legislation be passed requiring that ships be equipped with wireless, and that a continuous watch be kept. He was called for the Titanic's maiden voyage, but had to beg off for personal reasons. All this is well-treated in the program.
My interest was in the wireless apparatus. I had known roughly about these things (as we all do) but had never thought more precisely on the subject. The program showed apparatus and procedures that excited my curiosity. The presenters of the program were quite obviously only superficially acquainted with the technical aspects, and could give no exact information. Somewhere there are people who know these things, who have the circuit diagrams and relics of the equipment, but at the moment I have no access to these treasures. Therefore, I will give my best shot at interpreting the technical aspects and hope for future enlightenment.
When we telegraph down a wire, we launch electromagnetic waves that are guided by the wires and the earth. Our instruments respond rather to the conditions existing behind the wavefront than to the wavefront itself, but the speed of communication is that of the guided electromagnetic wave propagation, which is a bit slower than the speed of light. On a cable, with its high capacitance, the speed is considerably less than that of light. If we place a circuit near our telegraph wire, we can detect waves passing by induction, but the effect decreases rapidly with distance, and reflects only changes in the circuit, not steady conditions. If a higher, carrier frequency signal is used, signals can be transmitted by induction, but only over short distances.
Clerk Maxwell showed theoretically, and Heinrich Hertz demonstrated experimentally, that a fluctuating current could launch electromagnetic waves that were independent of any conductor or the earth. These waves became weaker by spreading, but otherwise there was no reduction in intensity (in the absence of absorption). Hertz's waves were in a frequency range we would now call microwaves, many MHz, and he showed that they could be refracted, reflected and polarized exactly like light waves. Indeed, light waves are electromagnetic waves, and Hertz was mainly trying to show this.
Hertz's transmitting and receiving apparatus consisted of a Leyden jar (a capacitor), a loop of wire, and a variable spark gap. One jar was charged up, and the spark gap reduced until a spark occurred. A sympathetic spark then was seen at the gap of the other apparatus, showing that energy had been exchanged. The most interesting thing that was discovered here was that the spark discharge was oscillatory. When the spark gap broke down, a column of ionized air now completed the circuit. As long as this conducting column existed, the circuit containing capacitance C (the Leyden jars) and inductance L (the wire, perhaps wound into a coil to increase the inductance) was complete, and the capacitance started off fully charged. The result was a decaying sinusoidal oscillation that continued for many cycles. The natural, or resonant, frequency of the LC circuit was the frequency of oscillation in this case.
The circuit by itself (perhaps with no air gap to make things easier) responded to the same frequency if an electromagnetic wave fell on it. These LC circuits were called tank circuits, because they would store energy for short times. They are fundamental parts of transmitting and receiving circuits. In the days of the spark gap, there was little need for selectivity, but a great need to store up the limited energy available so it could do something. A resonant antenna becomes electrically much larger than its physical size, and is able to draw in a large amount of wave energy.
Detecting electromagnetic waves was quite difficult, requiring strong waves, until the discovery of the coherer opened things up. The coherer was a tube filled with particles, probably magnetic, that normally provided an open circuit or a high resistance, but in the presence of a rapidly oscillating current cohered, or clumped, providing a low resistance. It was like a relay contact operated by the high-frequency current. Since the particles tended to stay clumped, a vibrator was attached to de-cohere them. The current controlled by the coherer could operate, through a relay, a Morse register or similar device. The coherer was very finicky and unreliable in operation, so there was a vigorous search for an alternative.
The program showed the spark gap, which was mounted on a Ruhmkorff induction coil (or something similar). I think this apparatus had a DC winding, which was supplied by batteries and keyed. When the current rose to a certain level, an electromagnet caused contacts in the DC circuit to open, and the current collapsed. This cycle repeated itself, as in an electric doorbell. Around the DC winding was the secondary winding of many turns. When the DC field collapsed, a large potential was induced across the ends of this winding, which were connected to the air gap. This served to break down the air gap, exciting oscillation in the tank circuit. The oscillatory nature of the spark-gap circuit had been used by Hertz in 1887 to generate high-frequency electromagnetic waves. The high-frequency was blocked by the high inductance of the secondary winding, and confined to the tuned circuit. When the key was pressed, a series of these pulses was produced at the interruption frequency until the key was released.
At the receiver, the operator had earphones coupled to the receiving tank circuit. When a signal excited oscillations in the tank circuit, the operator could hear the coarse spark-gap signal. There was no way to generate a beat frequency at the time, so the irregularity in the signal was used instead as modulation. The buzz of a weak signal could be detected in earphones before it was strong enough to do other work. A signal strong enough to operate the coherer (only producible by shore stations) would allow the signal to be recorded by a register. The register shown in the program seemed to be a typical European land-line inking register. It was shown printing (or having printed) a short bit of tape which I could not read in the time available.
If a crystal (not a very sensitive detector), Fleming valve (a thermionic diode rectifier) or the new deForest Audion was used, A buzz was heard in earphones and reception was acoustic. The spark gap transmitter signal was inherently modulated at an audio frequency, which was much more distinctive than the on-off action of the coherer. When vacuum-tube oscillators were finally devised, heterodyne detection was possible. The received signal and the oscillator signal were mixed, and their difference was an audio signal that could be easily detected, even with a pure rf signal.
The spark-gap transmitter was so characteristic of wireless telegraphy that it gave its name to everything connected with radio in German--the operator was a Funker, and the process Funktelegraphie. Marine operators were called "Sparks." Early vacuum tubes--before the 1920's-- could not generate the high power required for long-distance communication across the ocean. The high-frequency Alexanderson alternator, which could supply kilowatts of pure rf, was developed by General Electric around 1918 for the purpose, and was used for radiotelephony as well. Thermionic vacuum tubes could reach much higher frequencies, and replace the alternator when they could handle the necessary power.
Wireless telegraphy depended on an unsuspected property of electromagnetic wave propagation. The scientific opinion at the time was that wireless telegraphy would be impracticable over any distance because the waves would simply rise above the earth. This is quite true for microwaves, and would be equally true for HF waves if it were not for the ionosphere. But Marconi used low radio frequencies, say 100 kHz, and for these it happens that an antenna mainly exites waves that are guided by the conducting earth or sea. These waves not only bend over the horizon, but also spread less than three-dimensional waves. Arnold Sommerfeld soon gave a theoretical explanation after Marconi's success was evident.
Throughout the program, a model of SS Republic frequently appeared, and quite evident was the antenna stretched from bows to stern. Oddly, this feature was never mentioned. It happens that a circuit radiates better the larger it is compared to a wavelength. Land stations could use high masts and extensive antennas, but at sea this was not possible. With Marconi's 100 kHz, the wavelength is around 2 miles, so any shipboard antenna was necessarily electrically short and a poor radiator. In addition, the horizontal polarization that was necessary was probably not as good as vertical polarization.
Some points of procedure were also mentioned. The distress signal CQD was carefully pointed out. This incident was said to be the first time it was ever used at sea. Since SOS replaced it in 1912, it had a short vogue. SOS was adopted by the International Radiotelegraph Conference in Berlin on 22 Nov 1906, according to a newspaper item. If this was indeed SOS (not CQD), the new signal was apparently not used immediately. The meaning of CQ, familiar to all radio amateurs, was somewhat scrambled, rather than simply stating that it was an invitation to all stations to transmit. The keys looked a little uncomfortable, but perhaps they had to handle larger DC currents than a normal telegraph key. Since Marconi was from Italy, what we now call International Morse was the alphabet used.
Composed by J. B. Calvert
Created 20 December 2000
Last revised 23 November 2001