Built on Facts

Yesterday I went to a cookout at a friend’s house. As usual when there’s lots of good food outside, soon enough a large number of flies decided to crash the party. There’s not much you can do about it. Chemical sprays will get into the food, so there’s the most effective option out the window. Flyswatters will work, but only if you’re willing to have smashed fly bodies everywhere. Citronella candles? I’m not sure if they work on flies at all, but even so the allure of fresh medium-rare beef might overwhelm the repellent qualities of the citronella oil.

But this family had something I’d never heard of before. Not just a flyswatter - an electric flyswatter. Press the button on the side and swing it at the fly in the air; there’s a spark and a pop, and the fly falls out of the sky like a little brick. No mess, and dementedly entertaining (I’ll admit it). Yeah it’s a little redneck, but effectiveness is effectiveness. Here’s a picture of a similar model I found online:

There’s rows of wires (presumably adjacent wires have opposite charge), and the fly that touches two has gone to the great fly beyond. The whole thing is powered by two AA batteries. How does it work?

Beats me. I don’t want to cheat and find out on Google. If it were my flyswatter I’d take it apart and find out. But since we like physics, how about we at least develop a hypothesis for the time being? Here’s mine: two AA batteries produce 3 volts. That wouldn’t (as the proverb goes) hurt a fly. Since the thing sparks some fraction of a millimeter when a fly touches it, I’d guess the voltage is in the single kV range. How can you get this from batteries which produce DC? Well, so as to not waste electricity I’d guess there’s a capacitor storing the energy. But a 3 volt capacitor of any size will not do anything to a fly. Therefore something has to increase the voltage, but a transformer requires AC current. So there’s probably some component rapidly switching the voltage on and off in short quick pulses which are then fed either through a tiny transformer or through a voltage-multiplier circuit. Maybe both! That puts a high-voltage charge on the capacitor. The capacitor has one end connected to the even-numbered wires and the other end is connected to the odd-numbered wires. The fly completes the circuit and the cookout is saved.

I don’t know if I’m right, but this is nothing new in the science business. Nevertheless, it’s a very interesting piece of physics kitsch! Science and engineering have given us everything from unfathomably sophisticated computers to space probes exploring billions of miles away (and a brand new one on Mars). Never let it be said the little things don’t get attention too!