Julius Sumner Miller: Lesson 37 - Miscellaneous Adventures in Sound

A - Since oscillating systems have intimate connection with the production of SOUND we show here a very pretty thing - THE BLACKBURN PENDULUM. A rigid support has two strings fixed to it which join a third to constitute a pendulum of interesting complexion. A funnel hangs as a pendulum bob. We put salt or sand or sugar in the funnel - draw the pendulum aside from its equilibrium position - and let it go. Sand pours out and the pendulum bob executes interesting gyrations. We have here compounded harmonic motions. The result is a figure of great beauty. These figures are called Lissajous Figures.

B - A glass tube has within it a metal rod. This metal rod is damped at its midpoint. In the glass tube we lay out some cork-dust. We now stroke the metal rod with a cloth bearing resin (or rosin) and an amazing thing ensues: The cork dust takes up very unique positions in the glass tube. What happens is this - briefly: The cloth with resin on it grabs the rod and lets go - grabs the rod and lets go — again our relaxation oscillation business. The compressional wave thus generated _m the rod "|umps off" the end of the rod - so to speak - travels to the far end of the glass tube -which is tightly closed with a stopper - and is reflected. This reflected wave or pulse joins hands with the incident wave to give rise to a standing wave - which we see in the configurations the cork dust takes up. And what can we do with all this? Answer: Determine the velocity of sound in the metal rod. This tube is known as Kundt's Tube.

C - An array of bar magnets hanging on strings permits the showing of a
compressional wave advancing. One magnet is pulled aside at the very end of the array and let go. It approaches its neighbor and drives it away - since the plarization is aligned this way. The pulse goes down this array - and suffers reflection from the ."open" end. It can be observed that a change in phase accompanies this reflection.

D - We explore once again the vibrations of high-voltage cables. Dampers are fixed to these cables to reduce the amplitude of oscillation. We show dampers made in the USA - in Germany - in Sweden.

E - The BULL ROARER: A slender strip of wood or masonite is fixed to a string at one end. We swing the strip in a vertical circle. A sound is produced. The strip catches up the air and pushes it to¬gether thus giving rise to a pressure pulse. The effect depends on Bernoulli's Principle - which see in an earlier Lesson in Mechanics.
We close this series on SOUND AND WAVES by showing the CHLADNI PLATES once again for here lies extraordinary beauty and drama. It is this point of view that I urge on all who study anything for then the subject becomes FUN and ENJOYABLE and the SPIRIT is lifted up.