Students run the test bay where scrapyard lifting magnets are rated. A bin of sand starts too heavy for the magnet to lift; sand drains out of it until the magnet suddenly can. The mass left in the bin at that moment is the capacity.
Data format
One row per test the student chooses to log: current, coil turns, gap, material, how the load was sitting, and the capacity in kilograms. Plot Capacity against Current with one line per coil to see the pull flatten off, then swap the horizontal axis for Gap, or group by Material or Presentation.
Model details
Capacity is driven by ampere-turns — current multiplied by coil turns — passed through a saturation curve, so the pull climbs steeply at low current and then flattens: the iron cannot carry any more flux. Doubling the coil therefore does the same thing as doubling the current. An air gap adds a lot of resistance to the magnetic path, so capacity falls off far faster with gap than students expect. How the load sits changes how much of it actually touches the magnet face, and the material sets the ceiling: aluminium is not magnetic and reads zero. Every reading carries ±3–5% measurement noise, so repeat runs cluster rather than repeat. The bin always starts at 1100 kg — above the strongest possible setup — and drains at a steady rate, so a weak magnet takes longer to reach its lifting point than a strong one. The shapes are physical; the absolute numbers are calibrated so a flat steel plate at full current lands near a real mid-size yard magnet's rating, not taken from any one manufacturer's chart.
Held constant: plate thickness. Every test plate is the same size and thickness, and only the metal differs. Thickness is a real variable — a plate too thin to carry all the magnet's flux lets some of it leak straight through and holds more weakly, and the pull rises with thickness until the plate can carry the lot, after which extra thickness adds nothing. It is left out here so the four variables on offer stay separable, not because it does not matter. Note it is thickness, not mass, that does this: mass mixes thickness together with area and density, and in a lifting problem a heavier plate is also harder to lift, which is a different effect pulling the other way.
How do I explore?
- Set up the magnet: how much current runs through it, and which coil is fitted.
- Set up the load: what it is made of, how it is sitting, and whether there is a gap under the magnet.
- Press Run the test. The magnet grips, but the bin is too heavy — so sand starts draining out.
- Watch the number fall. The moment the magnet lifts the bin, that number is the capacity.
- Press Log this run to keep it, then change one thing and go again.
What should I do/notice?
- Change only one thing between runs. If you change two, you will not know which one did it.
- Keep turning the current up. Does it keep helping by the same amount every time?
- Try 300 turns at 40 A, then 600 turns at 20 A. Anything odd about those two readings?
- Find a setting where the bin empties completely and the magnet still never lifts it. What would you change first?
- Before you press Run, say out loud roughly where you think it will lift. Were you close?
What about the data?
Every run you log adds a row with your settings and the capacity in kilograms. Plot capacity against the one thing you were changing and you have built that magnet's capacity chart — the same chart that ships with a real one.