English: Drawing of the acoustic apparatus called Kundt's tube, invented by August Kundt in 1866 to measure the speed of sound in gasses; also shows standing wave patterns created with it. From Kundt's original 1866 paper. The device (fig.6 & 7) was a horizontal glass tube (B,B') containing a small amount of fine powder. One end had a piston attached to the end of a metal resonator rod (A,A'), clamped at its center (K), excited by rubbing with chamois, so it radiated sound waves into the tube. The other end was blocked by a moveable piston (I) which could be used to adjust the length of the tube. When the tube was adjusted to resonance, the powder collected in piles at the nodes of the standing wave as shown in Fig. 3, at intervals of half a wavelength (λ/2), allowing the wavelength of the sound to be measured. Figs 1, 2, & 4 show more complicated patterns (striations) produced by the sound waves. Alterations to image: Rotated 90° clockwise