Crookes' Tube
UCL's Crookes' Tube is one of many variations of electrical discharge tubes used to demonstrate the properties of cathode rays in the 19th and early 20th century. The tube contains a flat metal Maltese cross on a hinge, which can be stood to show that cathode rays travel in straight lines. When an electric current is run through the tube, it would emit a colourful glow. A video of a similar tube in action is available below.
Fact File
| Object Category | Crooke's Tube |
| Collection | UCL Sciences and Engineering Collections (Medical Physics) |
| Accession Number | MEDPHYS-33 |
| Dimensions | Height 250mm, Width 300 mm |
| Geographic Region | Europe |
| Estimated Date | Post 1900 |
Crookes' 1878 Bakerian Lecture - Click to find out how he first presented his tubes
Object Biography of Crookes' Tube
It is not known where or when UCL’s Crookes’ tube was made; it is possible that it was blown at UCL by a resident glassblower, however it seems more likely that the tube was purchased as it has a manufacturer's mark printed on the top.The tube’s end-caps match those made by E. Leybold’s Nachfolger after 1914, suggesting that this could be the manufacturer. Clear glass to cover the electrodes was adopted in 1900, so it is likely the tube was made after this date.
At some point the tube was donated to or purchased by the Medical Physics department at UCL, whose inventory label is on the base. As a departmental item the tube may have been used for demonstrations, although it probably spent the majority of its life on display. How the tube reached the Sciences and Engineering Collection is also a mystery; it is likely that Medical Physics found the object and donated to the collection during a clear-out.The Crookes Tube is now housed in the UCL Sciences and Engineering Collection and is used for object- based learning. Its life has taken new direction through social media, appearing on Facebook as part of a ‘Material Matters’ project, allowing it to interact with audiences outside of UCL.
Visual Analysis
The most striking aspect of the object is a conical cylinder of transparent glass, with the smaller end narrowing and terminating in a metal cap from which extends a U-shape of the same metal. Joined to the bottom of the tube are two long glass knobs, one which fits into the stand and has an identical metal cap on its side, and the other which extends down, folds back inside itself, enters the body of the tube and curves into a thin S-shape. Irregularities in the thickness of the glass imply the tube is hand-blown, and joins are evident where the two glass knobs have been separately joined to the body. Attached to the glass S-shape inside the tube is a metal Maltese cross, which can be flipped up and down on a hinge. The cross is most likely made of aluminium and appears silver in colour.
Inside the tube extending from the caps is an electrode of about 1cm long, which consists of a wire coated in clear glass. The electrode near the base terminates in a bright silver metallic tip, whilst the other has a silver tip from which extends a stick of duller silver metal, ending in a flat plate roughly 1.5cm in diameter [Fig. 4]. The tip and wire are likely to be made of platinum, coated with lead glass. The wooden base in which the tube stands is of a turned, varnished, dark-coloured wood and has a ‘Medical Physics Inventory No.’ sticker on the underside.
Operational Principle - Structure and Function
As the Crookes’ tube is catalogued as a scientific object, it is likely the main motivation for its design was its scientific function. The tube is made of glass as it can withstand the force of the vacuum inside it, as well as allowing viewers to see the visual outcome of the experiment. The tube is made air-tight by the different components being fused together when hot, as is evident from the joins between the two knobs and the body of the tube. Different types of glass were employed to produce different coloured light, such as lead glass to fluoresce pale blue and uranium glass to fluoresce green. The tube could either be activated to discover which type of glass is used, or an analysis of the object using techniques from materials science could be carried out; uranium glass emits radiation, which could be detected using a Geiger-counter. The end-caps are made of metal to conduct electricity; it is here that the electricity supply would have been attached. The cross is on a hinge to allow it to be stood up when the experimenter begins, casting a shadow in the fluorescence. The loose hinge allows the experimenter to jolt the tube so the cross flips downwards and cathode rays hit the area of the tube which was previously in shadow, causing the area to glow brighter than the surrounding glass. Lead glass is used to cover the two platinum electrodes and to seal the tube, as the two substances would expand at a similar rate when heated. The base allows the tube to stand and separates it from any external wires, which Crookes warns may affect the charge within the tube when it is activated.
Although the structure of the Crookes tube is related to its scientific function, is was not only designed for science. The turned wood stand and choices of colour suggests that there was also an aesthetic concern to its creation, therefore there must have been a demand in the culture from which it rises for scientific objects with aesthetic value.
History of the Science
Cathode ray tubes perform an important role in the history of medical physics. The phenomenon of an egg-shape glass tube glowing when an electrical current was applied to it was recorded by Hauksbee in 1705, which was followed by Michael Faraday updating the equipment to cylindrical glass tubes in the early 1800s. In 1857 the instrument maker Johann Heinrich Wilhelm Geissler developed a new mercury vacuum pump, allowing brighter effects to be produced in the tubes. Among other scientists, William Crookes began experimenting on the rays in 1878, improving the vacuum that could be created in the tubes. For this reason, the tubes are often called ‘Crookes’ tubes’, although they are also called ‘discharge tubes’ or ‘cathode ray tubes’. Crookes presented his experiment tubes at the 1878 Bakerian Lecture in Sheffield, including a tube to demonstrate that;
‘the molecular ray which produces green light absolutely refuses to turn a corner, and radiates from the negative pole in straight lines, causing strong and sharply-defined shadows of anything that happens to be in its path’
(Crookes, 1878, p. 147)
He then adds that the ‘green phosphorescent light’ travelling in straight lines was also noted by H E Goldstein in 1876, thus Crookes was not the first to actually discover the phenomenon. However as Baird points out, scientific instruments are the embodiment of scientific theories, meaning that Crookes’ improved discharge tubes were as significant a development as the new theories. After the Sheffield lecture experiments with discharge tubes captured the interest of the public and Crookes tubes were mass manufactured and sold in trading houses for demonstration and scientific collection. Crookes later developed tubes which were used in the discovery of X-rays, and Cathode Ray Tube technology was even used in early televisions!
A Scientific Commodity
Examples of these tubes are not uncommon; many still exist in school and museum collections. For example the Science Museum houses multiple Crookes tubes, including a ‘Shadow Tube’ (SCM- Electricity and magnetism, Object No. 1914-77) similar to the one housed at UCL. As they were mass-produced, sold in trading houses and collected, it is likely that many of the remaining Crookes' tubes are, and have always been, used for display and not for scientific discovery. Their production was profit-driven, motivated by the demand for aesthetic curiosities from science enthusiasts. After Crookes gave his Bakerian Lecture in Sheffield in 1878 where he demonstrated the tubes, the demand for them exploded in the scientific and non-scientific communities alike; the tubes are aesthetically pleasing, making them popular with collectors, and are enchanting when they are activated to produce their 'magical' displays.
References and Further Reading
Booth, Nick, 2015. Discussion at Second Object Meeting, (Personal Communication, 15.02.2015).
Brenni, P., 2007. Uranium Glass and its Scientific Uses. Bulletin of the Scientific Instrument Society, Volume 92, pp. 34-39.
Brickerstaff, J., 2015. How did it all begin?. [Online]
Available at: http://www.neonunity.co.uk/index.php/12.
Crookes, W., 1879. Bakerian Lecture: On the Illumination of Lines of Molecular Pressure, and the Trajectory of Molecules. Philosophical Transactions of the Royal Society of London, 170(I), pp. 135-164.
Kingery, D., 1996. Learning from Things: Method and Theory of Material Culture Studies. 1 ed. Washington: Smithsonian Institution Press.
Maia, E., Serra, I. & Peres, M., 2010. The Gas Discharges in History and Teaching of Physics and Chemistry, Lisbon: University of Lisbon.
Nuffield Foundation: Practical Physics, 2011. Electron defletion tubes: straight line streams. [Online]
Available at: http://www.nuffieldfoundation.org/practical-physics/electron-deflection-tube-straight-line-streams.
Science Museum, n.d. Online Science: Crookes' shadow tube, 1914. [Online]
Available at: http://www.sciencemuseum.org.uk/online_science/explore_our_collections/objects/index/smxg-6581.
The Cathode Ray Tube Site, n.d. Crookes tubes. [Online]
Available at: http://www.crtsite.com/page7.html.
UCL Chemistry, 2015. Professional Services Staff. [Online]
Available at: http://www.ucl.ac.uk/chemistry/staff/support.
UCL Science Collections, 2012. Material Matters Facebook Page. [Online]
Available at: https://www.facebook.com/MaterialMattersObjectHandling/timeline.
Wright, A., 2007. Crookes Maltese Cross Tube. [Online]
Available at: https://www.youtube.com/watch?v=Xt7ZWEDZ_GI&index=20&list=UU78aAdJxJvaykPdyfTc12ww.