Arago's disc: a rotating magnetic field dragging an aluminium disc

The idea

A magnetic field moving relative to a solid conductor induces closed loops of current in it, and the force on those currents drags the conductor after the field without any contact, which is the principle of the induction motor and of the eddy-current brake.

Safety card

What you need

• Aluminium disc, 100 mm diameter and 1.0 mm thick (21 g), balanced on a needle pivot or a ball-bearing hub, with one radius painted for video
• Second aluminium disc of the same size carrying eight radial slits cut from the rim to within 10 mm of the hub, edges filed smooth (cut by the teacher or the laboratory technician beforehand)
• Acrylic disc of the same diameter and thickness as a non-conducting control
• Neodymium disc magnet, grade N42, 20 mm diameter by 10 mm long, mounted on a plate with its centre 35 mm from the rotation axis
• 12 V DC motor on a variable bench supply with the current limit set, or a hand crank, driving the magnet plate beneath the disc
• Clear guard enclosing the whole rotating assembly, with a collar securing the disc to its hub
• Acrylic spacer sheets 2.0 mm thick to set the magnet-to-disc gap at 2.0, 4.0 and 8.0 mm; vernier caliper
• Phone recording slow-motion video at 240 frames per second, or an optical tachometer; stopwatch
• Magnet keepers, eye protection, and a copper disc of the same size where one is available

How to do it

1. Set the magnet plate so the gap to the solid aluminium disc measures 2.0 mm on the caliper with the magnet's centre 35 mm from the axis, fit the guard, and check by hand that the disc turns freely on its bearing.
2. Run the magnet at about 5 rev/s and record 10 s of slow-motion video. Read the magnet's rate and the disc's rate from the painted radius and compute the slip. Repeat at about 10 rev/s and at up to 20 rev/s, three runs at each rate.
3. Replace the disc with the acrylic one and repeat at 10 rev/s, then with the radially slit aluminium disc, and record each disc's rate against the magnet's.
4. Return to the solid disc, raise the gap to 4.0 mm and then 8.0 mm with the spacers, measuring each gap on the caliper, and record the disc's rate and slip at 10 rev/s, three runs at each gap.
5. Reverse the coupling to measure the drag: with the motor stopped and the magnet held still at a measured 2.0 mm, spin the disc by hand to about 10 rev/s and record its rate against time on video until it stops. Repeat at 4.0 mm and 8.0 mm, three runs at each gap.
6. Fit an exponential decay to each run and take its time constant. The drag coefficient is the disc's moment of inertia divided by that time constant. Compare the three coefficients with the ratios the magnet's shape predicts, 1 : 0.598 : 0.190.
7. Remove the motor, hang the magnet on a thread above the disc, spin the disc by hand and record which way the magnet turns.
8. Where a copper disc of the same size is available, repeat step 5 at 2.0 mm and compare its drag coefficient with the aluminium one.

What you should see

The solid aluminium disc turns the same way as the magnet and settles just short of the magnet's rate, because the drag torque has only the bearing friction to overcome: at the modelled coupling of 1.94e-3 N m per rad/s a bearing torque of 0.05 mN m holds the slip at 0.026 rad/s, under 0.005 rev/s, so at 10 rev/s the disc and the magnet look locked together on slow-motion video; a real coupling below that upper limit gives a larger slip, in inverse proportion to the coupling. The acrylic disc of the same size and mass distribution carries no induced current, so it does not follow the magnet; any slow drift it shows comes from air stirred by the magnet plate. The radially slit aluminium disc turns far more slowly and lags well behind, because the slits, cut in from the rim, cross the loops the induced current would make beneath the pole, so each loop must close inside one 45 degree sector or round through the uncut ring near the hub. Raising the gap from 2.0 mm to 4.0 mm drops the drag at a given slip to 0.598 of its value, and 8.0 mm drops it to 0.190; these two ratios follow from the magnet's shape alone and do not depend on its remanence, which is what makes them the robust test of the model. The decay runs give the drag coefficient directly, as the moment of inertia divided by the time constant: a time constant of 1.0 s, for example, would correspond to 2.7e-5 N m per rad/s, some seventy times below the modelled upper limit. A measured coefficient is expected below that limit, because the model takes the field on the magnet's axis across the whole pole face and lets the induced current return round the pole with no resistance of its own. A copper disc of the same size should give 1.64 times the aluminium coefficient, the ratio of the two conductivities the model uses. Spinning the disc with the magnet held still slows it to a stop without contact, and with the magnet hanging free the disc drags the magnet round in the same sense, which is the same coupling read the other way.

What changes

What you change

rotation rate of the magnet, then the magnet-to-disc gap, then the disc material

What you measure

steady rotation rate of the disc, the slip, and the drag coefficient from the decay time constant

What you keep the same

• same magnet and the same 35 mm radius for its centre
• same bearing and the same disc diameter, thickness and balance
• gap measured on the caliper before every run
• same starting rate for every hand-spun decay run
• room temperature recorded, because conductivity falls as the metal warms

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• The disc follows the magnet because aluminium is magnetic; aluminium is not attracted to a magnet at rest, and the drag appears only while the field moves relative to the disc.
• The disc must reach the magnet's rate exactly; the torque is proportional to the slip, so some slip always remains and it is what produces the torque.
• Slitting the disc weakens it mechanically and that is why it drags less; the slits work by interrupting the current path under the pole, so the same metal at the same mass drags far less.
• A rotating field needs a rotating magnet; three fixed coils fed with currents a third of a cycle apart give the same rotating field, which is how an induction motor turns with no magnet moving inside it.
• The braking force grows without limit as the disc speeds up; at high slip the induced currents cut the net flux through the disc and the torque falls away again.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-13PH11/12-2PH11/12-3PH11/12-5
• Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-02PY-12WS-02PY-12WS-03
• Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

1. www.nsw.gov.au/sites/default/files/noindex/2025-03/physics-stage-6-syllabus-2017.docx
2. education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/physics/12Physics_-module-6-guide.docx
3. curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes
4. web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/72.39.html
5. case.edu/artsci/phys/courses/demos/eddy.htm