There are many zapper on the market. Dr. Clark mentions that many variations in zapping technique have been discovered since the first one found around 1990. The original technique showed that very small animals like our parasites, could be killed with a very small voltage. But only IF the voltage is 100% Positive offset, and only IF the voltage is varied up and down repeatedly. If the varying voltage becomes NEGATIVE, even momentarily, it supports and maintains their lives! (page 83 – prevention book)
In earlier books a circuit was described that produced a totally Positive electrical field at all times, called “positive offset”. But many zappers were built with small substitutions when the exact components were not available. This often brought the resulting electrical field too close to negative so that brief excursions into the Negative field were inevitable.”
Therefore it is important when buying a zapper that it is 100% positive offset at all times, even better if 0.25v, square wave.
Prices vary, it all depends on what one wants and how much he wants to spend.