Tras is right. It's SO simple to make with three 9 volt batteries or a small alternator setup (if anyone has any clue about simple building). Putting a silver coin in milk used to keep it from souring quite as quickly, but I wouldn't think it would give you levels high enough to be used as a potent antibiotic. And that's precisely what properly made colloidal silver is.
And PLEASE- NEVER, EVER use anything except .999 or higher pure silver. A "silver coin", if it's an old silver dollar or "junk" silver, isn't pure silver, and you sure don't want "colloidal heavy metals" in your body! Even silver can be overdone... but most of the others are downright toxic.
I have rechargeable 9 volt batteries in the preps, but even though I make gallons of colloidal silver every year, a set of 3, 9 volt batteries lasts me at least 6-9 months. Sometimes quite a bit longer.
Summerthyme