The "Rijke tube" seen in the photograph at the left below, sometimes called a "hoot tube," is a glass tube with a nichrome heating coil in the tube about one-quarter of the way up the tube, as seen in the photograph at the right below. When it is activated by passing electrical current through the nichrome wire, it responds by emitting a loud tone, which in this case is the fundamental frequency of the tube acting as an acoustical open tube. Click your mouse on the photograph at the left below to hear the nice sound.
If we heat up the nichrome wire so that the Rijke tube emits its tone, then rotate the tube so that it is horizontal:
- (a) How will the loudness of the tone change, if at all?
- (b) How will the frequency of the tone change, if at all?
What will then happen when I return the tube to its initial upright orientation?
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The answer for part (a) is: the tone will cease, as can be observed in an mpeg video by clicking your mouse on the video below.
This is because the source for the tone consists of random noise generated by convection currents in the tube, caused by the nichrome coil. When the tube is oriented horizontally strong convection currents are precluded, so no strong noise source is available. When the tube is returned to its vertical orientation the tone sounds once again, because the convection currents can recur, as long as the nichrome coil remains sufficiently hot. Any time the tone sounds it will be at the same frequency, because the length of the tube does not change.
In practice, this tube must be turned slightly more than horizontally before its convection currents are totally eliminated and the sound ceases. When returning the tube to its upright position, a temporary very slight increase in the frequency can be heard by the astute observer. This is as yet unexplained.