The Cathode Ray Tube

Experiments with electric sparks in glass tubes containing a near vacuum had been carried out by Hauksbee and later Cavendish and Davy. Francis Hauksbee ¹ had demonstrated a “mercurial phosphor” or electrical glow in an evacuated column of glass before the Royal Society in 1703 and followed it up with a series of experiments that led to a new method of producing large quantities of static electricity using a glass globe which was spun rapidly under a hand or piece of wool or other materials. If the globe was evacuated it gave a greenish glow. .² [Fig. 1 shows a demonstration of Hauksbee’s electrical machine]

Sir Humphrey Davy demonstrated the carbon arc lamp around 1802 and discovered in experiments conducted during 1821-22 in which an electric current was passed through an evacuated glass chamber, that

“When the minutest quantity of rare air was introduced into the mercurial vacuum, the colour of the electric light changed from bright green to sea green, and by increasing the quantity, to blue and purple. At a low temperature the vacuum became a much better conductor. … At temperatures below zero the light was yellow and of the palest phosphorescent kind, just visible in great darkness, and not increased by heat.”³

By heating the mercury he found that the intensity of the glow increased, leading him to suppose that the light was due to the mercury vapour.⁴ This is the basis of the Mercury Vapour lamp that we use in some street lighting today.

Various experimenters continued to work with electric discharges in rarefied atmospheres. Plucker in Germany had concluded that the cathode rays were waves and William Crookes in London published a paper in 1879⁵ in which he argued that the “dark space” in the glow discharge that occurs with increasing vacuum in a glass tube was caused by the flight of a stream of “molecules” (as he called them, they were subsequently called “corpuscles” by J.J. Thomson, and we now know them as electrons⁶) from the negative electrode to the positive electrode and when these corpuscles hit the glass surface at the end of the tube they caused it to glow. If a piece of mica (usually in the shape of a “Maltese Cross”) was interposed in the stream then it caused a shadow in the phosphorescent glow on the tube-end demonstrating that the corpuscles travelled in straight lines [Fig.2], and if a magnet was brought near the tube it would draw the stream towards it.⁷ These two properties of the electric discharge in an evacuated tube needed only to be coupled with the knowledge of phosphorescent materials to complete the basis for what we know now as the CRT or Radar or Television tube.

By 1896, the properties of cathode rays were reasonably well known and had produced one major new development with Roentgen’s discovery and very thorough characterisation of X-rays. The German physicist Ferdinand Braun was keenly aware of this and decided to carry out an investigation into the possible use of the cathode rays. He knew that the glass of the vacuum tube itself was phosphorescent, and that the rays themselves travelled in straight lines which could be deflected by a magnet. So, in early 1897, he had a tube constructed in which a phosphorescent material was painted onto the inside of the end of the tube opposite the cathode. An electromagnet was wrapped around the neck of the tube and when an alternating current was applied to it and the cathode was heated, a stream of electrons could be seen to draw a thin glowing vertical line onto the screen. Braun’s solution to getting the horizontal display was to use a rotating mirror in front of the screen.⁸ It took several more years for a second electromagnet to be wrapped around the neck of the tube and a horizontal deflection to be added so that a full 2-dimensional display was achieved.

The photoelectric effect, in which the resistance of a piece of Selenium varied considerably depending on its exposure to light, was discovered by Willoughby Smith (an English telegraph engineer) in 1873.⁹ This discovery contributed to the important idea that one could transmit pictures by electric telegraph [see the section on the Facsimile and the transmission of images, above, p.20]. The first attempts to produce an electric camera were carried out by one Denis Redmond who, in February 1879, wrote to the English Mechanic and World of Science about his experiments with selenium arranged as the rods and cones in the retina and “scanned” sequentially by setting the camera lens to focus on each selenium cell sequentially.iii He didn’t get it to work but he had the right idea. There were a number of other approaches to sending images down a wire but they been canvassed above. What is of importance here is the development of the display device that came out of Braun’s work on the oscilloscope.

In summary of this section, we can see that many of the sometimes wild and speculative ideas of the natural philosophers and electricians from the 17th through the 19th centuries, as they were turned into working mechanisms and made commercially viable, provided the seeds for the development of much of the technology that made the 20th century into the Information Age. We can see that the philosophical speculations of Leibniz’ binary number system and his Ars Combinatoria, the electrical experiments of the natural philosophers and telegraphers and the automation of the manufacturing technologies of the weavers over the 18th and 19th centuries, converged into the notion that a machine could change its character through the substitution of new sets of instructions, represented in both automata and the Jacquard loom, and, through the work of the 19th Century Irish mathematician George Boole,¹⁰ that logical propositions could be represented in the binary form and operations carried out on them by mechanical and electronic machines. It is these ideas that form the basis of general computing. The other aspect of this experimental age that is so important for the story I am telling is that these notions also made possible the electrical representation, transmission, display and, ultimately, digital storage and manipulation of images and sounds. It is this convergence that brought us television, video, electronic sound, computing, computer imaging and, ultimately, the Internet with its ease in communicating information and images almost instantaneously around the world.

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FOOTNOTES

1 Hauksbee was working for Newton and reproduced some of Newton’s experiments in which he built a form of electrical machine using an evacuated rotating glass globe. See ‘sGravesande, W.J. (1721) Mathematical Elements of Natural Philosophy, confirmed by Experiments. Or an Introduction to Sir Isaac Newton’s Philosophy. London: J. Senex and W. Taylor, pp.10-11. and Plate 1.
2 Hauksbee, Francis, (1709) Physico-mechanical experiments on various subjects, containing an account of several surprizing phenomena touching light and electricity, London: R. Brugis, 1709; Mottelay, 1922, op cit, pp.149-51. and The Bakken: A Library and Museum of Electricity in Life: Francis Hauksbee the Elder ( – 1713?) <http://www.thebakken.org/artifacts/Hauksbee.htm>
3 Davy in the Encyclopædia Britannica, – eighth edition. quoted in Mottelay, 1922, op cit, p.345. Davy continues “… When the vacuum was formed by pure olive oil and by chloride of antimony, the electric light through the vapour of the chloride was more brilliant than that through the vapour of the oil; and in the last it was more brilliant than in the vapour of mercury at common temperatures. The light was of a pure white with the chloride, and of a red inclining to purple in the oil. … In carbonic acid gas the light of the spark is white and brilliant, and in hydrogen gas it is red and faint. When the sparks are made to pass through balls of wood or ivory they are of a crimson colour. They are yellow when taken over powdered charcoal, green over the surface of silvered leather, and purple from imperfect conductors.”
4 Guillemin, 1891, op cit, pp.430-31.
5 Crookes, William (1879) “The Bakerian lecture. On the illumination of lines of molecular pressure and the trajectory of molecules.” Philosophical Transactions of the Royal Society of London. 170. [2] pp.135-64.
6 The Germans argued that the cathode rays were waves because they were found to be able to penetrate thin sheets of Gold, while Crookes argued that they were particles, which J.J. Thomson subsequently proved until they were shown to also be waves by his son G.P. Thomson in 1932.
7 Guillemin, 1891, op cit, p.439; and Thomson, J.J. (1907) The Corpuscular Theory of Matter, London: Archibald Constable & Co., pp.3ff. See also <https://web.archive.org/web/20060507105518/http://members.chello.nl:80/~h.dijkstra19/big/crookes/maltesecrossbig-2.jpg > for a photo of the Crookes tube with a Maltese cross element and the actual discharge glow.
7 Kurylo, Friedrich and Susskind, Charles (1981) Ferdinand Braun – A Life of the Nobel Prizewinner and Inventor of the Cathode-Ray Oscilloscope, Cambridge, Mass.: The MIT Press, pp.88-91. See also Katz, Eugenii, (2003) biography of Karl Ferdinand Braun at <http://chem.ch.huji.ac.il/~eugeniik/history/braun.htm> The paper is Braun, K.F. (1897) “Uber ein Vehrfahren zur Demonstration und zum Studium des zeitlichen Verlaufes variabler Strome” (On a Method of Demonstrating and Studying the Time Dependence of Variable Currents), Annalen der Physik und Chemie, Leipzig, vol. 60, no.1, p.552.
8 Smith, W., letter to Latimer Clark, 4 February 1873, Cited in “Effect of Light on Selenium during the Passage of an Electric current”, Nature, 20 February 1873, p.303. Quoted in Lange, A. (2003) Histoire de la television at <http://histv2.free.fr/selenium/smith.htm>
9 Redmond, Denis D., (1879) “An Electric Telescope”, English Mechanic and World of Science, no.724, 7 Feb. 1879, p.540. Quoted in Lange, A. Histoire de la television at <http://histv2.free.fr/19/redmond.htm>
10 Boole, 1854, op cit.

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