Soda Straw Damping Demo


In seismology a pendulum is often used to detect ground motion.  The pendulum must have very low resistance to movement, but should quickly stop swinging when the ground motion stops.  This is best achieved by damping the motion of the pendulum.  The ideal damper will produce a force on the pendulum that resists motion in direct proportion to the velocity of the pendulum.  One way to do this is by attaching a plunger surrounded by oil to the pendulum.  Obtaining the correct viscosity oil can be difficult and oil can be quite messy to deal with.

An alternative is to use magnetic damping.  When a permanent magnet and a conductor are in relative motion, an electric current is generated in the conductor.  This current, in turn, generates a magnetic field, just as an electro magnet does.  The magnet field produced by current in the conductor always opposes the magnetic field of the permanent magnet.  Thus the relative motion is opposed by a force that is proportional to the relative velocity.  This is exactly the force that is needed for damping a seismic pendulum.

This demonstration uses a small, cylindrical, rare-earth magnet, a soda straw just slightly larger than the magnet, and a piece of copper sheet.

3/16" diameter by 1/4" long N42 rare-earth magnet
(K&J Magnetics, Inc.)                                                    25 for $3.75

5"x7"x0.016" Cu sheet (K&S Engineering)                                $6.49

Soda Straw with inside diameter just slightly larger than the magnet.    Cell-O-Core, 1/4" diameter, 7 3/4" long Jumbo Straw JW4                                       

Using a vice, a hammer, and rod of similar diameter to the straw, I bent the end of 3.5" x 7" piece of the copper sheet into a tube.  I cut the tube from the rest of the sheet and adjusted it a bit more so that it would slip easily, but with little clearance, over the straw.

I slid the magnet into the straw and stapled each end shut, and slid the copper sleeve over the straw.

To block the end of the straw, to keep the magnet within, I used short rings cut from 7/16" O.D. brass tubing.  K&S Engineering, Stock # 130. 7/32 x 0.14 wall.

12" length, $1.69

The ring, about 3/16" wide, is cut from the tube with a fine jewelers saw, after sliding the straw in place.

Burs are removed.  Then the magnet is inserted and the steps are repeated to place a ring in the other end.

Each end is dipped into a drop of super glue to hold the ring in place.

This is the final setup, with a copper sleeve in place.

Click on the image for a short video showing the magnet sliding through the straw.

Click here for a larger image of the straw.

Note that because the force on the magnet is proportional to its velocity, rather than accelerating it will reach a constant terminal velocity while traveling through the section of the straw surrounded by copper.

Here's a magnetic damper design for a vertical pendulum.