MRIs and just about any wireless device are just some examples of breakthroughs that wouldn't have been possible without the discovery of superconductivity and Guglielmo Marconi's early wireless telegraphy experiments. The two recently were named IEEE Milestones in Electrical Engineering and Computing, and both were honored this month.
Superconductivity is a property exhibited by certain materials of electrical resistance at temperatures close to absolute zero, and superconducting materials exhibit near-zero electrical loss. That makes possible large electromagnets, high-current-carrying power transmission cables, and electrical machinery that can operate efficiently without any dissipation.
Superconducting magnets have made possible magnetic resonance imaging and other medical diagnostic applications. They have played a crucial role in high-energy physics particle accelerators such as the Large Hadron Collider at CERN in Geneva. Superconducting magnets have also made possible high-speed magnetic levitation trains, prototypes of which have already been built in Germany and Japan, and commercially operating versions have been opened to the public in China, Japan, and South Korea.
Researchers continue to find new applications for superconductivity. It is being evaluated as a way to make more efficient electricity grids by enabling improved power transmission, low-loss power transformers, and fault current limiters. Superconductivity has the potential to be used for digital logic chips with clock frequencies that are at least an order of magnitude faster than what's possible with today's semiconductor technology.
Such applications are around today because of an unexpected discovery that happened a century ago by a professor at Leiden University, in the Netherlands.
Heike Kamerlingh Onnes established a research program in the late 1880s at Leiden University to liquefy helium, which at the time was the only known gas yet to be liquefied. Onnes succeeded in 1908, and for that feat, he received the 1913 Nobel Prize in Physics. His discovery unexpectedly led to superconductivity.
He spent the next few years studying the electrical, thermal, and mechanical properties of materials at temperatures that could be found only by using liquid helium as a refrigerant. One research area focused on the resistivity—the measure of how strongly a material opposes the flow of an electric current—of very pure metals at ultralow temperatures. While studying the resistivity of a mercury sample on 8 April 1911, Onnes and his colleagues—Cornelis Dorsman, Gerrit Jan Flim, and Gilles Holst—observed that when the temperature was lowered beyond minus 269°C the resistivity dramatically dropped to "practically zero." They went on to show that by cycling the temperature around that critical point, they could reproduce the cycle of the sample between the zero resistance—or superconducting state—and the normal-conducting state. That indicated the transition was a real physical phenomenon. As it turned out, Onnes and his fellow researchers had the opportunity to discover superconductivity because theirs was the only lab until 1923 that had liquid helium.
Onnes disclosed his research in a 1911 paper, "On the Sudden Rate at Which the Resistance of Mercury Disappears." Initially he called the phenomenon "supraconductivity" but later adopted the term "superconductivity."
A ceremony was held on 8 April to unveil the Milestone plaque on the 100th anniversary of the discovery. The plaque was placed in the building on the Leiden University campus where Onnes made his discovery. It states:
On 8 April 1911, in this building, Professor Heike Kamerlingh Onnes and his collaborators, Cornelis Dorsman, Gerrit Jan Flim, and Gilles Holst, discovered superconductivity. They observed that the resistance of mercury approached "practically zero" as its temperature was lowered to 3 kelvin. Today, superconductivity makes many electrical technologies possible, including magnetic resonance imaging (MRI) and high-energy particle accelerators.
Marconi's first experiments in wireless telegraphy were aimed at communicating without wires at increasingly long ranges. He began experimenting in 1894 in a lab located in the attic of his family home, Villa Griffone, in Pontecchio, Italy. But researching wireless signals in the so-called Silkworm Room proved too limiting for what he set out to discover. So he took the instruments—the same that were being used at universities to experiment on electromagnetic waves—and relocated them to his garden to explore signals at long distances.
The idea of wireless telegraphs was not new; researchers had been experimenting with them for more than 50 years, but none had been commercially successful.
During the next year, Marconi went on to develop a telegraph system that could transmit signals from his garden over Celestini Hill, which was almost 2 kilometers away. Marconi had not discovered any new principles but had instead assembled and improved a number of components, and unified and adapted them to his wireless system. It used a simple oscillator, a wire placed at a height above the ground, a coherer receiver that was modified from French inventor Edouard Branly's original device (also an IEEE Milestone) to be more sensitive and reliable, a telegraph key to operate the transmitter to send short and long pulses (for Morse code), and a telegraph register that was activated by the coherer, which recorded the received signals.
Early in his experiments Marconi had been able to transmit the signal over only short distances. He was able to extend the system's range after increasing the length of the transmitter and receiver antennas, using lower frequencies, and constructing the transmitter and antenna vertically while positioning the antenna to touch the ground. After transmitting the signal over the hill in 1895, a gun was fired to let Marconi know it had gone through.
A ceremony honoring the Milestone was held 29 April and a plaque was placed at both locations. One plaque reads:
In this garden, after the experiments carried out between 1894 and 1895 in the "Silkworm Room" in the attic of Villa Griffone, Guglielmo Marconi connected a grounded antenna to its transmitter. With this apparatus the young inventor was able to transmit radiotelegraphic signals beyond a physical obstacle, the Celestini hill, at a distance of about 2 kilometers. The experiment heralded the birth of the era of wireless communication.
The other plaque states:
On this hill, during the summer of 1895, the radiotelegraphic signals sent by Guglielmo Marconi from the garden of Villa Griffone were received. The reception was communicated to Marconi with a gunshot. This event marked the beginning of the new era of wireless communication.
More information about these and other IEEE Milestones can be found on the IEEE Global History Network.