The transistor as a voltage amplifier

Principle

This experiment serves as an introduction to the application of transistors for amplifying alternating voltage.
The stray field produced by the transformer of the power supply unit induces a small alternating voltage in the coil. It is so minimal that it can not be sufficiently detected directly with the headphones. To attain the necessary voltage amplification, the alternating voltage is superimposed on a base voltage produced by a voltage divider. This alternating voltage controls the collector current periodically around the average value set by the bias voltage. The fluctuations in the collector current produce voltage fluctuations in the headphones, evidenced by a humming noise.

This experiment also serves as an introduction to the much discussed "electromagnetic pollution" issue. It proves that electrical devices produce magnetic fields in their surroundings when being in operation.

Task

How can electromagnetic pollution be detected?

Try to amplify the small alternating voltages induced by alternating magnetic fields of the power supply unit in a coil so that they can be detected on headphones.

Literature for this article als follows

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