Apparatus Competition

2006 AAPT Summer Meeting

Syracuse, NY


An Invisible Force


Richard Heckathorn

14665 Pawnee Trail

Middleburg Hts, OH  44130



geepaw ‘at’ wowway ‘dot’ com



A falling aluminum disc through a strong magnet field is observed to take a longer time to fall a given distance than it takes the same aluminum disc fall the same distance outside a magnetic field. Observation also indicates that the aluminum disc falls at constant speed in the magnetic field while analysis of the falling aluminum disc outside a strong magnetic field shows that it is accelerating as it falls. 

Construction of Apparatus: 

1. Cut two pieces of  Plexiglas, one 2 x 3 x 1 inch (base) and

    one 3 x 6 ½ x 1/2 inch for the vertical support.

2. Glue the two pieces of Plexiglas together.

3. Fasten the three magnets, one above the other to the vertical support.

4. Obtain some aluminum discs that will pass through the magnets.

Use of Apparatus: 

1. Drop the aluminum disc and watch it fall.

2. Describe how it falls.

2. Next drop the disc through the magnets.

3. Describe how it falls.

4. Does it fall any different through the magnets compared to falling on its own?

5. Finally drop three discs (side-by-side) together through the magnets.

    (Observe the falling discs both from the front and the top.)

6. What do you observe? Do they fall together? Do some get through the apparatus

    before others?

7. Additional: Obtain and drop discs of other materials through the magnets


This apparatus demonstrates electromagnetic induction. As the aluminum disc falls through the magnetic field of the magnets, a current is induced in it. This current, present in the magnetic field gives rise to an upward (magnetic) force on the aluminum disc. This force, opposes the force of the earth on the aluminum disc (gravity). The result is a reduction of the net force on the disc and a reduction in the acceleration of the coin. Visual observation indicates that the aluminum disc falls with constant velocity indicating that the net force quickly reduces to zero.


This apparatus can be used in class to demonstrate electromagnetic induction. The best use is to place the apparatus on a desk where students can get to it. They will quickly pick up a disc and drop it. The amazed look on their face is worth it all. Giving the students a day to research how it works is then followed by an explanation.


This apparatus can be used in conjunction with:

1. dropping a neodymium magnet through a copper or aluminum pipe

2. swing a copper or aluminum paddle through a strong magnet

3. putting a twisting copper ring into the mouth of a magnet

Source of Materials:

The three large computer hard disc magnets* were obtained from a friend for $10.00 each. The pieces of Plexiglas was obtained from scrap from the school shop. The six bolts were obtained from a hardware for 90 cents. The aluminum discs were found in a box of metal odds and ends.


*My friend has indicated that the large magnets are no longer available. Thus I will bring along for display, an apparatus made using smaller magnets taken from present day hard drives that anyone can quickly make.

The picture on the left is 6 ½ inches tall and the one entered into the competition.


The picture below is 3 ½ inches tall. The magnet for it came from a regular size hard drive.
















Gene Easter first showed me the magnet and how metal dropped through it would slow down. I then constructed the apparatus using the magnets.