Hi everyone,
This is a neat experiment that shows us that every coil has a self inductance.
In this example we will use the relay but you can also use the HT transformer of the Lectron set.
What happens here is that, when we press the switch the relay is energised which is shown by the light bulb.
As soon the switch is released, the relay coil gets de-energised and the collapsing magnetic field causes a current flow in opposite of the battery current.
The diode is mounted in such way that the meter indicates this so called flyback pulse.
The 1,5K resistor protects the meter.
Many greetings
Here some pictures :

