Truth be told, Bobby had no idea how to actually use the device in his hand. Actual Dark detectors weren't a thing they'd used in his first two years, preferring to do more theoretical rather than practical work, and with a much broader focus on creatures, their detection and defences, as well as some general spellwork.
It had no buttons or switches on it, its surface otherwise smooth - other than the dial and the metal eyes. Clearly, then, it required some external influence - he had his dad's words ringing in the back of his mind, that "nothing made is made to work accidentally", so it must have some triggers somewhere.
He tapped the box with his wand and muttered "Aparecium!" quietly, and a small brass handle revealed itself on the one side of the box. Bobby thought idly of the wind-up handles of gramophones like the one his grandparents had.
The metal eyes opened lazily, as if awakening out of a deep sleep, and blinked at Bobby in turn from 'left' to 'right' as he looked at it, which he supposed was in order of past, present and future, and all three stared at him for a moment - in as much as three clearly manufactured eyes on an otherwise inanimate object can 'stare' at someone. Bobby hoped, at least, that these were as manufactured as they looked. The notion of three formerly-owned-by-presumably-deceased-bearer eyeballs troubled him.
The first eye rolled around, in directions that were viscerally uncomfortable to a young man who was used to eyes only going a small distance... before the eye looked at him and closed. The second eye rolled around, back on itself, into its socket much like the first had, then it angrily blinked and stared with some alarm at a box next to Professor Montgomery's desk.
The third eye rolled around, more lazily than the first two had, with far less vim and vigour, rolling only once back in its socket rather than the several times the others hand, before staring vaguely in the direction where Professor Montgomery was standing.
Finally the gauge moved from end to end, energetically at first, before settling approximately two thirds along its scale, just within the 'yellow' section but before getting to 'green'.
Then a modest buzzing noise occurred - it was decently loud but not ear-splitting, of a middle pitch. Sort of like a telephone ring but continuous rather than ringing.
Bobby assumed that meant he had successfully turned it on and that it was picking up something in the here-and-now as prepared by the Professor and perhaps something in the future.
"Sir, I think it's picking up something by your desk, but I don't quite understand the other reading." It had occurred to him that that might not be the future as he'd supposed, but the past - and it was tracking Montgomery prepping the class. Or he was right about it being the future and it was a portent of something to come. Either way... better not to mention it, and hoped instead that the professor explained.