(Image: Thyristor Symbol)
(Image: How a Thyristor works)
If the wire course is touched by the loop, current flows into the 'gate' of the thyristor and the buzzer sounds.
The buzzer will continue to sound after the loop has touched the wire course. This is due to the thyristor which once activated cannot be deactivated until all power is turned off.
This type of circuit is also known as a ‘latching circuit'.