The Manometric Flame
Thomas B. Greenslade, Jr.
Kenyon College, Gambier, Ohio 43022
The manometric flame, first described by Rudolph Koenig in 1862, was the first device that enabled an acoustic signal to be turned into a visible signal. The sound waves are made to modulate the supply of gas to a small flame, and the resulting variations in the height of the flame are viewed in a rotating mirror. Today we can do the same thing with a microphone and an oscilloscope, but at a vastly larger expense.
In my first year of teaching at Kenyon College I discovered the device on the left-hand side of Fig. 1. It was 25 cm high, with an aperture on one side, and, on the other, a connection for a gas supply and a tip where the gas burned in a small flame. Between the halves was a flexible membrane. A Central Scientific Company catalogue from 1940 explained how the acoustic signal entering the aperture caused the membrane to vibrate. The stream of gas passing from the input connection to the flame was thus modulated at the same frequency as the sound wave, and the height of the gas flame oscillated. This motion was too rapid to be followed by the human eye, but viewing its reflection in the rotating mirror spread it out into a horizontal series of images, making it possible to see the variations in the height of the flame. In the spring of 1965 I demonstrated this to my class of about 45 premedical students, and this may very well have been one of the last time that students anywhere saw this system in action. (But see the last line of this article.)
Fig. 2, from an 1883 edition of Ganot’s Physics1 shows what students observed in a rotating mirror. The upper figure shows a fundamental tone and the lower one a tone at twice the frequency.
The developer of the manometric capsule was Rudolph Koenig who was born in 1832 in Koenigsburg in what used to be East Prussia. After study at the University of Königsberg (which awarded him an honorary Ph.D. in 1868), he was apprenticed in 1851 to the Parisian violin-maker, Vuillaume. In this work he showed an unusual aptitude for mechanical work, and an excellent musical ear. In 1858 he went into business for himself as the designer and maker of acoustical apparatus.
For the next forty three years, until his death in 1901, Koenig produced a series of instruments for the production and analysis of sound. These were manufactured and tested in his suite of rooms in an apartment house on the Isle Ste. Louis in the Seine River in Paris, a location chosen for its quietness. Dayton C. Miller, who visited Koenig in 1896 and 1900, noted that Koenig, who never married, lived in the small front room of his apartment, which was also his office and stock room. The building of instruments was done in the back rooms of the apartment by Koenig and a few assistants. No piece of apparatus was sold unless it was thoroughly tested, and perhaps used in an ongoing experiment, by Koenig himself.2
The manometric flame and the rotating mirror in Fig. 3 are the best examples in my collection. The capsule, a little over 30 cm in height, is marked, “W.A. Olmsted Scientific/Company/Manufacturers/Chicago.” And there lies a rather grim and gruesome tale. 3 In 1878 W.A. Olmsted had gone into business as a maker and seller of scientific apparatus, and had prospered. The Olmsted company occupied the sixth floor of a seven-story building on Wabash Avenue in Chicago. On a lower floor was a piano manufacturer, which used quantities of wood and volatile solvents and finishing materials. On March 17, 1898 a fire started in the piano company, which spread upward. In the fire, Olmsted and eight of his employees died. Soon afterward the company was wound up and its good will was bought by the Chicago Laboratory and Scale Company. This company was in turn purchased by the H.C. Stoelting Company of Chicago, and I often see pieces made by this company in my scientific travels.
The rotating mirror in Fig. 3 was made by the firm of Max Kohl of Chemnitz, Germany. It is one of the largest that I have seen, with mirrors 20 cm on a side. The massiveness of the system makes it easy to keep it rotating at a fairly uniform rate. The rotating mirror was first used in a scientific application by Charles Wheatstone (1802-1875) in his researches on the speed of electricity in a wire. In a paper published in 1834,4,5 a spark from a Leiden jar jumped across a gap at the beginning of an insulated wire a half mile long. On the way back, the electric signal produced a spark in a second gap. The two sparks were viewed in a rotating mirror, and by knowing the rotation rate of the mirror and the length of the wire, Wheatstone was able to show that electricity travelled with an appreciable fraction of the velocity of light. Today we can repeat the experiment with an oscilloscope to measure the small time intervals.
The mirror in Fig. 1 was made by the L.E. Knott Apparatus Company of Boston, and is listed at $9.75 in the 1916 catalogue. The catalogue has a drawing of the device with the mirrors removed, and inside you can see a rudimentary governor; to keep the speed constant, two small pieces of metal flew outward from their pivots as the mirror rotated. The base contains a small direct current motor whose speed can be controlled by a rheostat built into the base. The rotating mirror in Fig. 4 is also by Knott, and here a horizontally- mounted and rather large motor drives the mirror through a worm gear. The rheostat is mounted in the heavy base of the apparatus.
Two other manometric flames in the Greenslade Collection are shown in Fig. 5. The one on the left-hand side is simply a graceful and rather handsome piece of apparatus, but the one on the right-hand side was manufactured by a rather unusual maker of “scientific” apparatus. Dr. Lyman D. McIntosh (1849-1892) founded a company to manufacture electrotherapy apparatus in 1879 that existed until 1952. In 1889 it changed its name to the McIntosh Optical and Battery Company,6 and that is the name stamped on the wooden manometric flame capsule. Other McIntosh apparatus that I have seen included Newton’s rings, a tangent galvanometer, a milliammeter, a seven-in-one apparatus and a half-model of a steam engine. This is a small sample compared to the three thousand pieces of apparatus that I have photographed.
The manometric flame apparatus in Fig. 6 is a hybrid, probably made in the early years of the 20th century. Sound from the loudspeaker at the top is projected downward toward the horizontal manometric membrane at the bottom of the apparatus. Illuminating gas enters at the fitting on the left-hand side and burns at the top of the small vertical pipe on the right. The device was made by Kipp of Delft in the Netherlands, and is in the Greenslade Collection. This firm was founded in 1830, and my collection has a selenium cell made by it.
Koenig manufactured a number of organ pipes with manometric flame capsules attached to show the locations of nodes and antinodes.7 The pipe in Fig. 7, at Yale University, is typical. This is a stopped pipe; the solid plate across the top forces a node there. In Sound and Music by Zahm8 there is a discussion of the same apparatus with an open top. In this case there are antinodes at the top and the bottom, and the manometric flames there oscillate up and down, while at the middle there is a node, and the flame does not flicker. The pipe is effectively one half wavelength long.
With the top closed, a node is forced at the top. There is an antinode at the bottom, and the pipe is a quarter wavelength long. The frequency is thus half of that of the open pipe. Blowing harder on the pipe produces a higher frequency in which three-quarters of a wavelength fits into the pipe, and the resulting third harmonics three times that of the fundamental; this pipe produces only odd harmonics. The three manometric flames allow the student to observe the nodes and antinodes as the pipe is blown gently and then harder.
The resonance apparatus designed by Koenig is shown in Fig. 8. There is a thorough discussion in Zahn,9 along with a diagram that shows that a Helmholtz resonator was originally attached to the tube at the lower left-hand side of the apparatus. Blowing across the end of the open end of the Helmholtz device produced white noise, but its sharp tuning (my experiments with Helmholtz resonators have given Q values of the order of thirty) means that essentially only one frequency is fed into the system. The sound is directed through both sets of tubes, one of which has a fixed length and the other slides up and down in a manner akin to the mechanism of a trombone. On the output side are three manometric flame capsules, one for each of the sets of tubes and one being fed by both of them. As the length of the adjustable tube is changed, the latter manometric flame shows alternately a maximum signal and then a minimum signal. From the changes in the length, the wavelength of the sound can be determined with considerable precision. This apparatus was photographed at the University of Toronto in 1984, and it is part of the collection of apparatus that Koenig brought to the 1876 Centennial Exposition in Philadelphia.
Today we would call the massive piece of apparatus in Fig. 9 a Fourier analyzer. A complex wave shape produced by a musical instrument or the human voice singing a note is made up of a fundamental sinusoidal oscillation, plus overtones with frequencies that are integer multiples of the fundamental. These are known as Fourier components, and the trick is to find the amplitudes of these components, relative to that of the fundamental. The instrument consists of a series of Helmholtz resonators which are adjusted to respond at the fundamental frequency and those of the overtones. The relative amplitudes are given by the amount by which a series of manometric flames respond.
There are fourteen adjustable resonators, and eight of these at a time can be connected to the eight manometric flames that are shown in Fig. 10. The flames are viewed in the long rotating mirror that is cranked by hand. The analyzer in the figures was in the collection at the University of Toronto, and I was told that it was very tricky to get all eight flames running. However, a recent, impressive video clip by Paolo Brenni shows this device in operation. You can find it at: https://www.youtube.com/watch?v=OHdL-65dkkY. This clip also shows the basic manometric flame operation.
- A. Ganot, Elementary Treatise on Physics, trans. and ed. by E. Atkinson (New York: William Wood and Co., 1883), p. 144. ↩
- Thomas B. Greenslade, Jr., “The Acoustical Apparatus of Rudolph Koenig”, Phys. Teach., 30 (1992), pp. 518-524. ↩
- Thomas B. Greenslade, Jr., “Apparatus Manufacturers in Chicago, ca. 1900”, Rittenhouse, 13 (1999), pp.16-19. ↩
- Charles Wheatstone, “An Account of some Experiments to measure the Velocity of Electricity and the Duration of Electric Light”, Phil. Trans. of the Royal Society of London, 124 (1834), pp. 583- 591. ↩
- Thomas B. Greenslade, Jr., “The Rotating Mirror”, Phys. Teach., 19 (1881), pp. 253-255. ↩
- Dean P. Currier, Guide to Electrotherapy Instruments and History of their American Makers (West Conshohocken, PA: Infinity, 2005), pp 212-274. ↩
- For a recent book on Koennig, see David Pantalony, Altered Sensations: Rudolph Koenig’s Acoustical Workshop in Nineteenth-Century Paris (Dordrecht, Heidelberg, London, New York, 2009). ↩
- J.A. Zahm, Sound and Music, second edition (Chicago: A.C. McClurg and Co., 1900), pp. 231-232. ↩
- Op. cit. ref. 7, pp. 295-299. ↩