Apparatus Competition

2006 AAPT Summer Meeting

Syracuse, NY


Force Between Two Current Carrying Wires Demonstrator


Zbigniew Roman Fracz

Oakland Community College

7350 Cooley Lake Road

Waterford, MI  48327



zrfracz ‘at’ oaklandcc ‘dot’ edu



This simple to make, easy to set up and easy to use lecture demonstrator presents attractive or repulsive force between two parallel current carrying wires. Accompanying equipment required to use with this apparatus can be found on the shelf of any school, college or university physics laboratory.
























Figure 1. Complete demonstration set up and apparatus details.

Figure 1. Complete demonstration set up and apparatus details.


Figure 2. Apparatus details.



Construction of Apparatus: 

Two pairs of alligator clips with predrilled holes are fastened with wood screws, about ¾” from the edges (A) of two wooden blocks 7” x 1” x 0.5” each (see Figure 2 and 1). Drill ½” diameter hole about ¾” from (B).  This allows the blocks to slide on a standard laboratory ½” diameter rod, attached to a cast iron base or clamped to a table. Two aluminum ribbons covered on one side with plastic tape are attached to the alligator clips with the plastic side of the ribbons facing each other. The ends of the ribbons should be rolled up with the plastic side on the inside, so both parts of the alligator plug are touching metal. Wires with banana plugs are connected to the alligator clips to allow parallel or antiparallel current flow. The best method for making ribbons is to put a 60 cm long sheet of aluminum foil on a table, carefully cover it on one side with 2” wide 3M Scotch Storage Tape and using a sharp utility knife and ruler, cut about ½” wide ribbons and trim them to the required length. Both sides look visually identical, so it is helpful to use a permanent marker and write “Plastic Side” on the ribbon’s plastic to allow proper installation (plastic side to plastic - to avoid short connection). The ribbon’s length is not critical, because sliding wooden blocks on the rod allows adjustment for proper tension. Longer ribbons give better result showing the magnetic force on each, but this requires a longer rod to hold the wooden blocks.

Use of Apparatus: 

The best results for showing magnetic force on each ribbon is to use a car battery as a current source connected in series with rheostat (about 4Ω resistance). Adjusted resistance, slightly about (0.2 → 0.5) Ω, allows enough current to create a magnetic force (to see ribbons really moving), and protect current source. Plastic tape on the aluminum ribbons protects circuit against short connection.

Current should be created for a very short duration during demonstration (about 1 second maximum), allowing ribbons and plastic to cool down. This time is enough to show the effect. Current flowing longer dissipates too much of heat energy, which will destroy the plastic on the aluminum ribbon. In this case replacing ribbons is necessary. It is good to have spare ribbons handy during the demonstration (just in case).

There is no switch used for connecting and disconnecting current. Demonstrator touches wires with banana plugs connected to the source directly to the alligator clips during demonstration and explanation.

For parallel currents in the same direction two jumpers are used on the top block and the bottom.  The power source is connected accordingly. For antiparallel currents, only the top jumper is used and the current is applied to the bottom contacts.

For each situation proper loose, not stretched tension for both ribbons should be created by sliding the wooden blocks on the rod, allowing the ribbon’s to have motion during demonstration. For antiparallel currents, the starting position could be with the ribbons touching each other.

Created torques on the blocks naturally protects them from sliding down, so the drilled holes cannot be too big (too loose), just enough to allow the blocks to adjust.

Motion of the wires (about 10 cm displacement) and the clarity of the current direction are visible from a very far distance. This allows observation of the demonstration and its effect to be visible in small and very large lecture hall.

The cost of the apparatus and the shelf space required is minimal which allows high and low budget schools to implement this demonstrator in everyday teaching practice.