Cutting a magnetic field with a wire

Class practical

Students should previously have inserted a magnet into a coil and measured the e.m.f. generated. This is a simpler version showing first of all only one coil (or loop) of wire.

Apparatus and Materials

• Mild steel yoke
• Copper wire, insulated with bare ends, 200 cm
• Magnadur magnets, 2
• Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)-

Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Attach two Magnadur magnets to the steel yoke with opposite poles facing each other.
2. Connect a long lead of insulated copper wire to the galvanometer.
3. Move the wire through the field between the permanent magnets.
4. Try the effect of a coil of many turns (see picture) and see how this changes the deflection.

Teaching Notes

Students will find that:

• There is only a current when the wire and magnet are moving relative to each other
• The faster the magnet or wire is moved then the greater the current
• The current changes direction when the relative motion of the wire and the magnetic field changes direction
• The effect is greater when the wire is formed into a coil (because there is more wire moving across the magnetic field)

This experiment was safety-tested in April 2006