A sensitive center zero projection meter is connected in series with a long cable. When the wire is introduced to the field of a magnetron magnet, the meter will deflect showing an induced current. When the cable is removed from the field the meter will deflect in the opposite direction.
Demonstration Instructions: Equipment:
Magnetron magnet, length of wire with banana plugs, and ammeter.
Plug the wire into the ammeter (the most sensitive is 25 uA). Loop the wire and pass it through the magnet, observing the needle. Don't get the magnet close to your wallet or electronic devices.
The area through which the magnetic field "flows" increases as the loop is placed in the field. Thus, the magnetic flux changes over time, inducing an emf, which in turn induces a current. The current always flows in a direction that "cancels out" the change in magnetic flux. When the loop is fully in the magnetic field there is no current. When it is pulled back out of the field the induced current is in the opposite direction, this time opposing the decreasing magnetic flux.
Row II, Row III
See chapter 29, starting page 993, University Physics, for information on electromagnetic induction.
Young, Hugh D., Roger A. Freedman, and A. Lewis Ford. Sears and Zemansky's University Physics: With Modern Physics, 12th ed. San Francisco: Pearson Addison-Wesley, 2008.