Machine 22Build an electromagnet← → step through the guidegantry · 64 cm · 36 cm span
Cells–
Current–
Ampere-turns–
Holds–
Balance–
What this step adds; in part B, live currentParts already in placeTwo soft-iron cores, 25 mm across, on a yoke, each wound with 200 turns of 1 mm copper: 400 turns in series, about 37 m of wire and 0.82 Ω. Each Daniell cell gives 1.1 V; with an assumed 0.5 Ω inside each, 4 cells drive 1.56 A, 624 ampere-turns. The model treats the whole iron circuit as one equivalent air length: B ≈ μ₀NI / (2g + ℓ/μr), with 0.3 mm of air and scale at each pole face at contact, 34 cm of iron at μr ≈ 1000, and no fringing; the pull is F ≈ B²A / 2μ₀ at each pole. That gives 0.83 T and a hold of about 272 N (28 kg) on 4 cells, and 78 N on one, under the 10 kg load. Lift the poles 1 mm off the keeper and the 4-cell pull falls to 44 N, which is why the magnet is lowered onto the keeper before it lifts. The balance is tared with the magnet on it. 1 drawing unit is 5 mm. This is a guide to how the parts go together, not engineering drawings. Never stand or reach under the load: it drops the instant the circuit breaks. Copper sulfate is poisonous; wear gloves and keep it from children and drains.
