Self-induction when switching a circuit off

Principle

In the last experiment, the students learned that self-induced voltage is formed in a coil when a direct current circuit is closed, and that this self-induced voltage counteracts the connected voltage. Now, they should learn that self-induced voltage formed when a circuit is opened has the same direction as the original (i.e. connected) voltage.
If the students can already predict the results of the first experiment based on their knowledge of the laws of induction and Lenz's Law, then they should carry out the experiment to confirm their prediction nevertheless. Furthermore, the second experiment should demonstrate that self-induced voltage can attain values that greatly exceed the original voltage.

Task

What effect does a coil have when the circuit is opened?

Investigate the direction of the self-induced voltage produced when a direct current circuit is opened.

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