Hulda R. Clark's discovery of the Zapper technique was done by accident, as per her book "The Cure for All Diseases" relating of how it was created by she and her Electronics Engineer son Geoff.
Dr Clark was researching bioresonant destruction of parasites in vivo, having documented bioresonant frequencies of many parasites already. Each parasite, she found, could be destroyed by inputting electrical energy at the same frequency that was used to detect its presence, but at higher power levels; levels were still not perceptible by the person, however.
Her long term focus was discovering a cure for cancer. But she had discovered many other interesting phenomena along the way. In the case of cancer, she had determined one of the key factors was the presence of a specific parasite in the liver, Fasciolopsis buskii, that in the presence of hard-to-detoxify isopropyl alcohol, was able to live its full life cycle in the body, instead of its normal stage involving in a snail on the ground, and getting picked back up later by the host. And one of its lifecycle stages produced large amounts of the same substance as was used as an indicator for cancer at the time. She hypothesized that the same cell-reproduction stimulating substance also caused human cells to reproduce rapidly if they were already in a challenged situation. Thus, she reasoned, destroying the F. buskii would be a cancer treatment. She had determined the bioresonant frequency of F. Buskii parasite, and had her son Geoff design a $25 device to be sold in all drugstores for everybody's medicine cabinet, useful as a preventitive so as to eradicate cancer in the world.
Geoff designed a lowest-cost generator of that bioresonant frequency, based around the common 555 integrated circuit, but to save money he omitted the output capacitor. That made the output waveform be a simple on-off voltge pulse of a single polarity, of around nine volts peak amplitude. And when Dr Clark tested the device and found it not only wiped out F. buskii in vivo, but also lots of other parasites in vivo at the same time, she being the free scientist, chased down the phenomenon and eventually chose the 30 KHz pulse generator frequency worked best as a general purpose parasite disrupter - she considered pathogens of all kinds to be parasites. She thought of it as analogous to the back yard bug zappers, so she called the device a Zapper. And the name has stuck. She did not know how it worked, but she did know that it did work and did so spectacularly well. She adopted it into many of her protocols for well being, each of which also involved identifying and removing some specific substance, usually a heavy metal, involving cleaning up a person's environment.
The Clark-type Zapper has found enormous popularity around the world. Although not approved yet for medical use by doctors in America, individuals are allowed to experiment with it on themselves or their pets. Its very evident efficacy has gained many devoted experimenters involving use of the basic concept and exploring beyond there too. Currently the most sophisticated instruments are built in Europe, and are quite expensive; but simper ones are home grown. Clark originally described ways to build a Zapper from scratch in her books, thus enabling the technique to become immediately useful for experimenters.