Digital Kenyon: Research, Scholarship, and Creative Exchange Faculty Publications Physics 2014 Apparatus Named After Our Academic Ancestors, III Tom Greenslade Kenyon College, [email protected] Follow this and additional works at: https://digital.kenyon.edu/physics_publications Part of the Physics Commons Recommended Citation “Apparatus Named After Our Academic Ancestors III”, The Physics Teacher, 52, 360-363 (2014) This Article is brought to you for free and open access by the Physics at Digital Kenyon: Research, Scholarship, and Creative Exchange. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Digital Kenyon: Research, Scholarship, and Creative Exchange. For more information, please contact [email protected]. Apparatus Named After Our Academic Ancestors, III Thomas B. Greenslade Jr. Citation: The Physics Teacher 52, 360 (2014); doi: 10.1119/1.4893092 View online: http://dx.doi.org/10.1119/1.4893092 View Table of Contents: http://scitation.aip.org/content/aapt/journal/tpt/52/6?ver=pdfcov Published by the American Association of Physics Teachers Articles you may be interested in Crystal (Xal) radios for learning physics Phys. Teach. 53, 317 (2015); 10.1119/1.4917450 Apparatus Named After Our Academic Ancestors — II Phys. Teach. 49, 28 (2011); 10.1119/1.3527751 Apparatus Named After Our Academic Ancestors — I Phys. Teach. 48, 604 (2010); 10.1119/1.3517028 Physics Northwest: An Academic Alliance Phys. Teach. 45, 421 (2007); 10.1119/1.2783150 From Our Files Phys. Teach. 41, 123 (2003); 10.1119/1.1542054 This article is copyrighted as indicated in the article. Reuse of AAPT content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 138.28.20.224 On: Mon, 12 Oct 2015 19:09:30 Apparatus Named After Our Academic Ancestors, III Thomas B. Greenslade Jr., Kenyon College, Gambier, OH y academic ancestors in physics have called on ure 1 shows an early 20th-century example of Arago’s disk. me once more to tell you about the apparatus that We can explain this Arago rotation by noting that the moving they devised, and that many of you have used in copper disk has eddy currents induced in it by the magnetic Myour demonstrations and labs. This article is about apparatus dipole, and the magnetic field resulting from these currents named after François Arago, Heinrich Helmholtz, Leon Fou- interacts with the compass needle. The instrument in Fig. 1 is cault, and James Watt. listed in the 1911 catalogue of W. G. Pye & Co. of Cambridge as “Arago’s Rotating Copper Disk” at £1.12.6, or about $8. I 1. Arago’s Disk. François Arago (1786-1853) belongs recently gave it a new belt and it works very well. to a group of post-revolutionary French scientists who did extensive work in optics, particularly in the wave theory of 2. Helmholtz’s Resonator and Coils. A polymath light and the study of polarized light. He started his scientific works, by definition, in many fields, and Herman Helmholtz career in 1805 with a continuation of the survey of the prime (1821-1894) is an operational definition of the term. He did meridian that ran through Paris, used to fix the length of the research and published in the physiology of human hearing meter. Earlier it had been laid out from Dunkirk to Barce- and vision, theoretical mechanics, electricity and magne- lona, and Arago, assisted by Biot, repeated some of the later tism, meteorology, chemistry, and mathematics. His original work and continued across the Mediterranean to Majorca. In training was in medicine, but after a short career as a mili- conjunction with Fresnel he showed that two beams of light tary physician in Potsdam, his work on the conservation of polarized at 90° to each other do not interfere, thus supplying energy, based on physiological, physical, and philosophical important evidence for the transverse wave theory of light. arguments, gave him an appointment to the chair of physiol- He became the director of the Paris Observatory, and in ogy at Königsberg. Later he was a professor at Heidelberg and 1838 suggested an experiment to compare the speed of light Berlin. in glass or water with that in air. If the latter were larger, this During his work on human vision in 1851, he invented the would provide the final key to the wave theory. Unfortunately, ophthalmoscope,3 which allows the interior of the eye to be he had poor vision and the experiments were carried out in studied; today this can be found in almost every physician’s the 1850s by Foucault and Fizeau.1 office. Much of the information in introductory textbooks Arago was a well-known politician, having been elected to about the structure and optical constants of the eye, the cur- the Chamber of Deputies in 1830. Later, as a cabinet minister, vature of the surface of the retina, and the ability of the eye he was behind the abolition of slavery in France and its ter- lens to vary its focal length (accommodation) comes from his ritories. In 1833 he made it possible for Louis J. M. Daguerre work. His book Physiological Optics, from the 1860s, can still to petition the French government for support for his work on be read with profit. photography (the daguerreotype). The equivalent book for acoustics, Sensations of Tone, In 1824 he discovered that the frequency of vibration of which Helmholtz published in 1862, establishes the fact that a compass needle increased as it was brought close to the the quality of a musical tone depends on the harmonic over- surface of a horizontal sheet of metal. The next year he found tones that are present and their relative intensities. In this that a rotating copper disk would cause a compass needle, work, he used sharply tuned resonators, of the form shown in originally at rest, to start rotating in the same direction.2 Fig- Fig. 2, to detect what frequencies were present in a complex Fig. 1. Arago’s disk from the Greenslade Collection. Fig. 2. A set of spherical Helmholtz resonators by Rudolph Koenig at the University of Toronto. 360 THE PHYSICS TEACHER ◆ Vol. 52, SEPTEMBER 2014 DOI: 10.1119/1.4893092 This article is copyrighted as indicated in the article. Reuse of AAPT content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 138.28.20.224 On: Mon, 12 Oct 2015 19:09:30 sound; these are now known as “Helmholtz resonators.”4 In the form of the device shown in the figure, these consist of a spherical shell containing a volume of air, a hole or neck in which a slug of air can vibrate back and forth, and a slender nipple that can be held in the ear canal (or, today, connected to a sound level meter). The enclosed volume of air acts as a spring connected to the mass of the slug of air, and vibrates in an adiabatic fashion at a frequency dependent on the density and volume of the air, its molecular composition, and the mass of the slug of air in the neck. Originally Helmholtz used wine and other bottles, but soon after 1860 he asked Rudolph Koenig of Paris, the famous maker of acoustical apparatus, to make metal resonators with specific dimensions. The tangent gal- vanometer in Fig.3 shows the use of “Helmholtz coils.” In this instrument, the current to be measured passes Fig. 4. Foucault’s disk in the National Museum of through the coils, American History at the Smithsonian Institution. which are oriented with their planes in the magnetic north- south direction. The compass nee- dle in the middle Fig. 3. Tangent galvanometer with of the instrument Helmholtz coils, made by Elliott Brothers responds to the vec- of London and in the University of tor sum of the hori- Mississippi Collection. zontal component of Earth’s magnetic Fig. 5. Watt’s external condenser from the Greenslade Collection. field and the B field due to the current; the current is propor- tional to the tangent of the angle through which it turns.5 The original design, from 1837, had a single coil, but the use of his devising to provide enough illumination to make pictures two parallel coils was first suggested by Helmholtz in 1853. through the microscope. Later he made the first (daguerreo- When they are separated by a distance equal to the radius type) photographs of the Sun, which clearly showed sunspots. of the coils, there is a surprisingly large region in which the His co-workers in this research were Fizeau and Arago. His magnetic field is reasonably uniform. 1850 measurements of the speed of light in air and water We don’t use tangent galvanometers anymore, but we do showed that the former was the larger. As we have seen, Ara- use Helmholtz coils whenever a large region of uniform mag- go had wanted to perform this experiment, but his eyesight netic field is required. For example, apparatus to measure the had become faulty and it was done independently by Foucault charge to mass ratio of the electron is always put inside a pair and Fizeau (1849). His development of what we now call the of Helmholtz coils. In experiments in which the magnetic Foucault pendulum started with the devising of a support for field of Earth must be eliminated, pairs of coils tilted at the lo- the wire that gave no preferred direction to it; this was first cal dip angle are used. tried out in his cellar and in February 1851 was demonstrated at the Paris Observatory. While he did not invent the gyro- 3.