According to Frequencygeneration.com:
Many Zappers out there seem to have some modification that in one way or another deviates from Dr. Hulda Clark's published design. For instance there are zappers out there that are digital, and Dr. Clark's design is Analog. There are several zappers out there that output a complete different frequency than what is published by Dr. Clark, and yet they still call their device a Hulda Clark Zapper.
There are only two Zapper frequencies that Dr. Clark has published to date. Dr. Clark's original circuit, the 30 kHz Zapper is the most famous. (30kHz can also be written as 30,000 Hz). Dr. Clark's latest published circuit called, the Food Zappicator has a 1000 Hz output.
So there you have it, the two choices are: 30 kHz or 1000 Hz. If you would like to experiment with the exact circuit as described in Dr. Clark's books, the "Frequency Generation" brand Zapper is a great choice.
According to Don Croft:
We challenge anyone to rationally explain why the cycle of 7/20 was recommended. We've never had a bad report from our "fellow researchers" (customers) regarding extended use of the zapper and we've worn it around the clock without any discomfort or symptoms. Anyway--Dr. Clark has finally come around to recommending long zapping sessions.
We made the zapper smaller and attached the electrodes to the box. We have had a great many responses from our customers confirming that this design, being more convenient and inconspicuous, gets adequate usage to do the job. The addition of subtle energies has even made it pleasant and energizing to wear a zapper.
Who will go through the daily ritual of holding onto the electrodes for the requisite 3 weeks, twice daily? Besides--the higher costs of most other zappers is out of range for many people.
Our goal is to mass produce these in order to reduce the cost further. We feel that the simplicity of this technology is difficult for most engineers and health professionals to comprehend.
We sell many zappers to folks who already have more expensive and complicated versions, because the ease of use of this one is hard to beat and it obviously does the same work and then some.