The silver coin in the canteen was a means of miminal purification and retardation of bacterial growth, which is why you see them shaking the canteen before drinking in some old westerns, but minimal is the key. Parts cost maybe $15 for the last one I made for someone a few months back, plus the .9999 silver wire for the probes/ electrodes, which you may have to order up. Gang/stack three good quality 9V batteries in series - that's negative to positive with one positive terminal on one end of the stack and one negative terminal on the other end, 27 volts. You'll need a package of 9V battery snap-on terminals, some silver solder suitable for electronics, some very small shrink wraps for your electrode connections and battery configuration splices after they are soldered, and one slightly larger shrink wrap to tie the electrodes together maybe half an inch above the ends of the smaller ones, because you are going to bend them apart to right around 3/8 " spacing from the neck to the ends, with the straight parallel wires long enough to keep the yoke/connections out of the water in at least a pint jar. A cable tie-wrap will do to keep your battery stack together. Some twin lead stranded like good quality speaker wire a foot long or a little more (but not too long due resistance/voltage drop), will do for your leads. Use a two wire plug, automotive or electronic, (preferably reversible so you can change positive and negative electrodes without desoldering when the positive probe shows wear, but you can always just cut, reverse, and splice the leads), so you can disconnect the leads when not in use to avoid shorting them out and ruining your batteries during storage. Stick them in a pint jar of (only!) distilled water for half an hour and your ppm will be just about right for consumption or topical use, but you can make it strong as you like. Don't use leaded crystal glass. If you want to make a little foam or cardboard top for your jar or glass so the electrodes don't touch the bottom, you will avoid the losing current across the surface of the glass the wires touch, with a few marks on the glass, and get better battery life, but no real big deal. I often just lean the probes so only one is touching the bottom. If they get "fuzzy" with particles, swish those off in your solution when it's done, then wipe off your probes with a paper towel to clean them. Don't use scrubbers because you are losing silver. Good batteries will make a lot of collodial before replacement, but you'll be able to visually tell when the process slows down, and can check with a voltmeter. A cheap VoM is around $3 when on doorbuster sale at Harbor Freight, and you should have one anyway. There are plenty of good, more sophisticated plans on the net, and you may of course add a suitable housing, on/off microswitch, battery indicator LED with a resistor circuit, etc., but cheap and quick is good, and so is simple. The last time I needed #10 99.99 % silver wire, it was readily available from CCsilver.com, but no doubt has gone up in price since the early 2000's.
Buying the stuff in liquid form is really expensive. If you mail or ship this device with three batteries ganged to anyone be sure to label it "medical device" so the dilrods don't suspect it is a b-b-b-b___.
Tras
"They who have put out the people's eyes reproach them of their blindness."
John Milton, 1642