Although I don't think that answers the question, I am curious, WHY do you have them stick their finger in the bottle? And at what point in the effect?
As for the rings in question, they look like therapeutic rings, and I am pretty certain that the only magnetic parts are the little circles you see on the inside of the bands.
This means that the magnetic force would be minimal compared to other rings on the market, where the entire band is magnetic.
If you are looking for a cheap magnetic ring, check out
http://www.supermagnetman.net/index.php?cPath=48
Although their rings are also sold as "therapeutic" ones, they are entirely magnetic, and are just as as strong as the Wizard PK rings, and available in ALL sizes, not just small medium and large.