Ruben's Tube and the Science of Dancing Fire
If you are looking for an awesome science experiment that will leave your spectators in awe, then search no more! The Rubens’ Tube is a hypnotic display of flame and sound that many people have never witnessed. While the experiment is not for the faint of heart (it involves flammable gas), this project is well worth your effort. With a passion for science and a little patience, you can be the proud creator of this scientific wonder!
Origins of the Rubens' Tube
The Ruben’s Tube, or standing wave flame tube, came about as an invention of Heinrich Rubens and Otto Krigar-Menzel, who were German physicists trying to determine whether it was possible to use fire as a tool to make sound waves visible. This question came about after a Dutch physics professor, Pieter Rijke, investigated the relationship between gases, fire, and sounds. In Rijke’s experiment, he used iron mesh within a large glass tube to create singing flames. When the glass was held over the fire, the glass would emit a musical tone which confused scientists. Later it was discovered that the sound was emitted due to the recurring motion of hot and cool airwaves. As the waves of air vibrated through the glass, sound resonated at the natural frequency of the glass tube.
Although Rubens is famously known for the invention of the Rubens’ Tube, his work with energy measurements of black-body radiation is equally as important. These measurements led Max Planck to discover the Planck black-body radiation law, which describes that physical bodies will spontaneously and continuously emit electromagnetic radiation with the greatest amount of radiation being only when a body is at thermal equilibrium. This discovery was the precursor to what we now know in modern physics as quantum theory.
Heinrich Rubens was born in 1865 and lived until 1922. He was a professor at the University of Berlin and a participant in the Solvay Conferences of 1911 and 1913. He received the Rumford Medal in 1910 and was nominated for the Nobel Prize (physics) three times between 1907-1909.
So What Exactly Is a Rubens' Tube?
The Rubens’ Tube is a device that is used to teach acoustical resonance behavior. It was first described by Heinrich Rubens and Otto Krigar-Menzel in 1905 in a paper called the “Flammenröhrefür akustische Beobachtungen” which was published in the Annalen der Physik.
The original Rubens’ Tube used a four-meter section of pipe that had about two hundred holes across the top which were evenly spaced apart. Both ends of the tube were sealed shut, flammable gas was pumped into the pipe, and a fitted speaker was attached to one of the ends. When the tube was filled with the flammable gas, there was only one route for the gas to escape through, thus creating a row of diffusion flames across the top of the pipe. Since the pressure inside the tube was equalized, the row of flames would stand at the same height. As soon as you play sound through the attached speaker, the height of the flames will change quite drastically. The height changes that you see is equivalent to the wavelength of the sound being played.
And How Does It Work?
According to physics, the sounds that we hear are audible vibrations that require a medium or material to travel through. In the Rubens’ Tube, the sound waves that we hear from the music are traveling through the flammable gas within the tube. As the sound travels down the length of the tube, localized pressure areas will occur. When these areas of pressure are higher, the gas will escape the surrounding holes faster, creating taller flames. When the area of localized pressure is lower, the gas escaping the surrounding holes will cause shorter flames. These high and low-pressure areas are created by the sound waves traveling through the gas. This is what creates the gorgeous height pattern and subsequently the visible sound wave within the flames.
Once you have very carefully put the flammable gas inside the sealed tube and ignited the escaping gas, you will see typical standing flames. Although these flames will flicker and move around slowly, they do not do much else. However, once you change up the pressure inside the tube with sound waves, that’s when the magic happens! The wavelengths of the sound change with the frequency of the pitch of the music, thus changing the height of the flames and how they dance.
When observing the Rubens’ Tube, you will see two sets of behaviors. The first is when the sound waves traveling through the tube are quiet. The quiet sound waves will have relatively low-pressure differences than the gas flowing through the tube. When the pressure decreases, the flow will also be reduced, thus providing shorter flames. On the other hand, when the sound waves are very loud, the changes in the localized pressure are much larger than the pressure being created by the gas, thus the gas gets propelled out of the holes at a faster rate. This causes you to see the very thin, long flames.
Building a Rubens' Tube
There are a number of great resources available for building your own Rubens' Tube. Instructables hosts an especially great article with a step-by-step guide and detailed information. The Colorado School of Mines is also a great resource. Just remember to be extra careful since this experiment involves flammable gas and fire!
Are you interested in making a Ruben's Tube?
© 2009 Micah