GoKawiilThe last several decades of technological progress have, in large part, been about finding more and more things we can do with semiconductors and the technology for producing them. Microchips have found their way into virtually every car, aircraft, appliance, and electronic device. Light-emitting diodes are steadily replacing older, less efficient methods of generating light (such as incandescent bulbs). Solar photovoltaic panels have become the most rapidly deployed energy source in history. Semiconductor lasers have enabled fiber-optic communication. Semiconductor-based charge-coupled devices (CCDs) and CMOS sensors are used for digital imaging. The list goes on.
But decades before the invention of the transistor, another enormous technological ecosystem was built around a device that manipulated the flow of electrons — the vacuum tube. In the first half of the 20th century, vacuum tube technology found its way into all manner of devices, from radios to TVs to the earliest computers. And like semiconductors today, vacuum tubes had applications far beyond electronic logic — the phenomenon they leveraged could be applied to everything from lighting and displays to video cameras and radars. And while the vacuum tube feels like an ancient technology that has long been superseded, much of the technological edifice still stands.
Origins of the vacuum tube
Triode vacuum tubes, via Wikipedia .
A vacuum tube is an evacuated tube (often, though not always, made of glass) containing electrodes, between which electrons flow. These tubes, along with various offshoots and technological cousins, were the product of two parallel strands of development.
The first line of descent was via what are known as “gas discharge tubes” — tubes where electricity is discharged through a highly rarefied gas (gas at very low concentration and pressure). Not long after the German scientist Otto von Guericke invented the first vacuum pump in 1650, early scientists began using such pumps to study highly rarefied gasses. It was observed that running an electric current through rarefied gasses could make them glow colorfully, but for many years this was mostly regarded as an interesting curiosity.
Glowing gasses in a gas discharge tube. A dark space can be seen between the glow near the cathode and near the anode. Via Wikipedia .
It wasn’t until the 1830s, with the experiments of the English chemist and physicist Michael Faraday, that the effects of electricity on rarefied gasses began to be studied more seriously. Faraday subjected a variety of rarefied gasses to electric current, observing their colorful glow, along with a curious “dark space” between the two electrodes. Faraday was a well-regarded scientist, and others took notice of his work: in 1855, Julius Plücker, a German scientist who “idolized Faraday”, endeavored to replicate Faraday’s experiments. To perform the experiments, Plücker obtained some highly evacuated glass tubes from the well-known instrument maker Heinrich Geissler. Geissler had built a vacuum pump capable of achieving a far lower vacuum than any pump previously, and Geissler’s tubes could function over a range of temperatures thanks to their platinum lead-in wires. (Platinum has nearly the same coefficient of thermal expansion as glass; several decades later, Edison would use the same strategy of platinum lead-in wires for this first incandescent light bulb.) These would later be known as “Geissler tubes”.
By using a series of well-crafted, highly evacuated glass tubes “of ever-escalating complexity”, Plücker followed Faraday’s footsteps in investigating the behavior of electrical discharge through highly rarefied gasses. During his experiments, Plücker observed what appeared to be some sort of emanation from the negative electrode (cathode). These emanations moved in a straight line, could be deflected by a magnetic field, and caused the wall of the tube near the positive electrode (anode) to glow green. Other scientists, including William Crookes and Plücker’s collaborator Johann Hittorf, investigated these emanations further, and they eventually came to be known as “cathode rays”. The tubes used to study cathode rays began to be referred to as “Hittorf tubes” or “Crookes tubes”, and proved to be an important scientific instrument for studying the nature of matter. In 1895, the German physicist Wilhelm Roentgen, using a Crookes tube to study cathode rays, stumbled upon X-rays, for which he would be awarded the first Nobel Prize in physics in 1901. In 1897, the British physicist J.J. Thomson discovered that cathode rays were actually streams of negatively charged particles — which were dubbed ‘electrons’ — for which he was awarded the Nobel Prize in physics in 1906.
Various 19th century experimental vacuum tubes, via Shiers 1974.
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