I just got a couple of zappers in the mail, a $10 zapper and an "X-Treme Triple Frequency" zappers from eBay, which was mentioned in a recent posting here.
I measured the output frequencies of both with a Fluke digital voltmeter, and was a little surprised that the $10 zapper was putting out 13.23 kHz, not 30 or above as expected. The voltage being delivered to the copper rings (which are basically thick, U-shaped pieces of bare copper wire which attach to alligator clips) with a fresh 9V battery is at about 2.5 V. I haven't used it too much yet. The frequencies on the Triple Freq are pretty close to 15 Hz, 2.5 kHz and 30 kHz, plus or minus.
The Triple Frequency zapper is reportedly wave-stabilized so that the wave stays square under load, i.e. when the handholds are being held. It tops out at a delivered voltage of about 4.5V with a fresh 9V battery. I first tried it last night, holding the copper pipes directly in my hands, and noticed pretty quickly that my hands smelled like copper even after several washings, and within a couple of hours, I could taste copper, like I had a penny under my tongue. Using paper towels over the handgrips has lessened this effect, and may be part of the reason this practice is recommended, above and beyond getting better contact.
I asked the manufacturer of the X-Treme model what is the maximum input voltage his circuit can handle, and he said 17V, so I've rigged up a cable which connects a 12V rechargeable sealed lead-acid battery (7 amp-hour) to the 9V clips in order to save my 9V alkaline batteries for mobile use. Be mindful if you decide to make this type of cable, because it's easy to get the polarity reversed, and circuits which have no protection against polarity reversal will zap themselves dead.
I'm definitely noticing effects of die off already, long zombie naps and not wanting to get out of bed, but I know from past experience that Herxheimer reactions are temporary and will try not to overdo it.