Back in about 1983 (or '84?), when Teknetics was still only a ~2 yr. old company, our club was somehow honored to have a Teknetics representative come speak to us. He brought their latest introductions of the time to demonstrate.
Bear in mind, that at this time, even though there's been a few rudimentary TID's machines (both them and Whites). Yet it was still a fairly new concept. Back in those days, word didn't travel as fast as it does with today's lightening age of the internet forums. So you were at the mercy of whatever you heard "through the grapevine" . Like if you happened to be hunting with someone who showed you some better mouse-trap. Or you were at the mercy of hyped up treasure magazine advertisements, etc..... So at the time this fellow came to talk to our club, the concept of tone ID, and bouncing needles for TID, was still un-heard of .
Up till this time , the only discrimination was ascending and descending. So if you knocked out tabs, you would miss nickels. If you knocked out square tabs, you'd miss round tabs. It was just assumed them, that whatever your setting was at, then anything underneath that, was GONE.
I distinctly remember the fellow waiving a wad of foil. It gave a distinctive tone. Ok, so what. Then he waived a nickel. It gave yet another distinctive tone, different than the foil wad he'd just waived. Hmmm. Then he waived a tab. It gave a 3rd different distinct tone. Hmmm. Then he waived a gold ring. It gave a 4th distinctive tone, DIFFERENT THAN THE PRIOR 3 ! Everyone's jaw dropped in the room. It was simply amazing. The dealer showed it with multiple different types of aluminum, and multiple gold rings, and you could plainly hear that the sound coming from the gold rings did not match his sample objects of aluminum.
There was immediate images of the club members of being able to go to parks, pass the junk, and dig "only gold". I mean, duh, how can you argue with this actual live demonstration ? A few guys in the room bought them on the spot, with this exact notion in mind. But lo & behold, they were in for a rude awakening, when they went to the nearest parks (which had been cherry picked leaving all the low conductors behind). They began to find out, that there is STILL a lot of aluminum that doesn't exactly fall into an isolated few foil samples. And an isolated few tab samples, etc...
I remember one guy tried an experiment: He collected all the aluminum he was getting, despite the notch outs. Then he notched ALL THEM out too. Then he went home, tried all his own collection of rings-found, and his wife's jewelry from her jewelry box. Only to discover he was now discriminating all that out too. doh !