Point at or tap any part to see its name. On the second setup, you can drag the magnet.
Amber dots show how much current flows. They do not show electron speed. Real electrons drift slower than a snail, less than a millimeter a second.
The dots move from plus to minus, the standard direction for drawing current. Electrons move the other way.
The numbers come from a teaching model of this bench setup. A real coil and magnet read in the same range, but not the same numbers.
Amp-turns is the number of turns times the current. The field in the coil grows with it.
The field is in millitesla (mT), figured at the center of a coil 4 cm across and 6 cm long. The level part of the earth's field, the part that turns a compass, is about 0.02 mT across most of the US.
A compass lines up with the earth's field plus the coil's field. Close to the coil, the coil's field is much stronger, so the needle points along it. Far away, the needle turns only partway.
The grey lines show the shape of the field, the way iron filings line up on the bench. One line is drawn for about every 25 amp-turns.
Each loop drawn on the coil stands for 5 turns. The wire is 18 AWG copper at 7.77 Ω per 1,000 feet, from Chapter 9, Table 8.
The lamp keeps the current down. The coil alone is close to a dead short across the battery. More turns means more wire, so the current drops a little.
The lamp is a 1156 car bulb, rated 12.8 V and 2.10 A. The model treats it as a fixed 6.1 Ω. The battery is a 12 V sealed lead-acid battery, held at a steady 12 V. A real bench setup has a fuse at the battery.
A real electromagnet has an iron core in the coil, which makes the field much stronger. This coil has only plastic inside.
The bar magnet is ceramic, 8 cm long and 1.2 cm square. Its material is rated about 0.4 tesla. The field at the end of the magnet is much lower. Slow moves it 10 cm a second. Fast moves it 20 cm a second.
The meter has zero in the middle, so it can swing either way. It draws almost no current, so the second setup shows no dots.
This meter reads tiny voltages. Testing a live circuit takes a meter rated for the job and training.
This is a top view of a bench. The parts are not drawn to the same scale.