Idiot. A continuation of the horizontal board surface behind the groove is a ledge. No ledge implies no groove.
Jonathan and Jon pretending to know what the video doesn't show is laughable. That is, the shape of the back of the disk and the distance between the bottom of the disk and the bottom of the groove. Do they have X-ray vision or ESP or is it mere assertion? The video shows an oblique perspective on the horizontal length of the wires and does not disclose what the length is relative to any circumference. Based on experiment, I estimated that the length along the wires is much closer to the circumference of the larger circle than the circumference of the disk. I am still waiting for the pretenders' reasoned argument, rather than mere wishful assertion, that the length along the wires equals the circumference of the disk.
If the circumference of the disk is as important to the demonstration as the pretenders allege, then why did the maker of the apparatus hide a crucial part of it and not put a mark on it like he did on the two circles?