I use only 1 speaker, and I kept it by one because of the next experiment. Acording to H Clark the speaker acts like a radio antena or a satalite dish, thats why only the positive lead is used. I changed the frequency to 1.6Mhz so that I can receive the signal with my AM radio. Now i was able to see how far I could receive this signal and use bigger and smaller speakers and even 4 speakers at once. The only thing that made a huge diffirence was increasing the power from the zapper. Using 4 speakers just changed the form of the signal, exactly like using 2 or 3 aerials on a two way radio making it directional. I found that the signal could be received up to 6 feet ( using a 4 ohm 40what 4 inch speaker ) Now arthur durksen stated 12 inches which is save but i would keep clear all metal atleast 6 feet on top and around to keep full power. If there is metal it would pull the field away from what you want to zapicate. Remember H Clark was a radio amature and these princeples come from there. Using 4 speakers in a box form will concentrate the signal more, but I don't think its strickly nesasary. And in my oreginal experiment with the milk i had 2 glases, the second glass was 2 feet away and it had the same results. My conclusion is this the breadbord zapicator with one speaker is sufficiant for household. If you want it stronger you can make your zapper stronger or play with diffirent frequency's.