Radio Transmitters, Early

According to James Clerk Maxwell’s theory, the very existence of electromagnetic waves entailed the principle that matter can act on other matter at a distance without any intervening matter. Before Heinrich Hertz’s experimental work with electromagnetic waves, no one was confident about this implication.

Hertz proved it so by detecting the wave generated by a spark from an electrical discharge across the gap formed by two electrodes. Hertz was driven to this experiment by the challenge posed by Maxwell’s theory, but as a theoretician himself, he had no interest in (and therefore could never have predicted) the application of his discovery—the spark transmitter and the early form of radio transmission.

Hertz was not the first to produce electromagnetic waves, but history regards him as the first to detect the waves he produced. In 1887 he created what he called ‘‘electric waves’’ by generating a sufficiently high voltage to create a spark. He detected the resulting electromagnetic wave by means of a loop of wire, which he called a ‘‘resonator,’’ the ends of which terminated in a small gap. When the wave passed through the loop, an electric current was induced in the wire causing a spark to leap across the gap.

Hertz’s discovery provoked much speculation about the use of electromagnetic waves for signaling, especially between land and ships in danger of running aground in fog. His apparatus, however, did not produce waves that traveled far enough or were stable enough for multiple simultaneous transmissions in the same locale. In fact, because Hertz analogized the propagation of electromagnetic waves to the propagation of light, he never would have overcome the limitations of his initial invention.

Early spark-gap transmitters consisted of a pair of electrodes and a combination of condenser and coil that were tuned to a certain frequency. The spark generated across the gap provided a short burst of electrical energy that shocked the tuned circuit into oscillation producing an electromagnetic wave that would eventually die out. Applying continuous shocks to the tuned circuit, however, would sustain the wave, much like a flywheel is kept in rotation by the continuous but intermittent force of a piston.

These transmitters produced waves that were inefficient, variable in frequency, and broad- banded. When a spark occurred across the gap, it caused some of the metal in the electrodes to vaporize. Being conductive, this vapor allowed an electrical arc to form across the gap, which lowered the efficiency of the transmitter and caused it to generate waves of multiple frequencies. Some means of dissipating the vaporized metal around the electrodes was needed—a process later called ‘‘quenching.’’

In 1889 Oliver Lodge demonstrated electrical resonance, the property that made possible the generation of a wave of one stable frequency. From that point the search was on to find the optimum combination of configuration and composition for the design of spark transmitter electrodes. Marconi’s designs included iron electrodes shaped like mushrooms. A variety of other metals went into the electrodes: silver, aluminum, graphite, tungsten, and others. In an effort to quench the arc, some electrode designers tried forcing the spark through running liquids or encasing the electrodes in water-cooled jackets.

Nikola Tesla and other researchers developed rotary gaps to produce sparks with higher repetition rates and thus more stable waves. Such continuous waves could produce musical notes; in fact, some early wireless stations in Great Britain were known to have transmitted electromagnetic renditions of ‘‘God Save the King.’’ As these designers produced purer sinusoidal waves, the prospects for transmitting voice seemed within their reach. No designer, however, could overcome the spark transmitter’s inherent noisiness in order to make radio telephony practical.

Spark transmitter designs did eventually make radio telegraphy practical. In 1897, Marconi secured a patent in Great Britain for a wireless telegraphy system that could transmit messages up to 3 kilometers. Most of the components of his system were adaptations of existing devices— Righi’s spark transmitter, a Branly coherer, and a Morse inker, among others.

Marconi’s contribution that broke the limitation on transmission distance was his decision to liken his wireless system to the existing wired telegraphy systems. Transmitters and receivers in a wired telegraphy system used an earth ground as one of their interconnections; Marconi grounded his sparkgap transmitter and its receiver, and thereby created the first practical wireless telegraphy system. Marconi went on to develop his spark transmitter system to produce a 5-centimeter spark from 100,000 volts that transmitted a series of Morse dots from Cornwall to Newfoundland—the first transatlantic radio transmission.

On the American side of the Atlantic, Lee de Forest and Reginald A. Fessenden were developing their own spark transmitter systems. De Forest’s design consisted of tungsten electrodes with an adjustable gap; Fessenden’s was a rotary spark-gap design. Both of these inventors attracted the attention of the U.S. Navy, whose patronage was a major force in the development of wireless telegraphy. At the same time they were also testing the Poulsen arc transmitter, which covered greater distances and was freer from noise.

Spark transmitters were pressed into service in the early years of the twentieth century in a variety of settings. The armed forces of many nations used them, and spark transmitters first played an important role in war in the conflict between Japan and Russia in 1904. Cruise and merchant ships at this time also used spark transmitters; the Titanic broadcast being one of the first distress messages in the form of SOS in 1912.

Ultimately, however, de Forest’s invention of the triode vacuum tube, or thermionic valve, in 1912 made possible the efficient generation of pure sinusoidal electromagnetic waves and rendered the spark transmitter with its noisy broadbanded output obsolete. In its brief lifetime the spark transmitter was essential for jump-starting the theory and practice of radio communication. Today, however, it is an historical curiosity, having been banned from the ether in 1927 by U.S law because of concerns about fire safety.