If you’ve ever constructed a Tesla coil or watched a demo, there’s something fascinating about the steady stream of purple sparks that spray out from its discharge terminal, the noise of the spark, the smell of the ozone. You’ll also likely realize you’re witnessing the step-up of a hundred volts or so into tens of thousands, and that the output of most coils (tabletop variety) is rather harmless, due to the limited current and high output frequency that channels most of the energy over the outer surfaces of living organisms — the so-called skin effect. The Tesla coil is also a radio transmitter, embodying elements found in any such device — a source of oscillations and resonant circuitry — and a good case can be made for Tesla’s having “gotten there” before Marconi. However, according to Nikola Tesla, his invention was not intended as a device for wireless communication, but rather as a means of generating very high frequency alternating current that might be used for wirelessly powering homes and industry. When the circuits of early radio transmitters (spark oscillators) and the Tesla coil are compared, there’s little difference, except for a coupling network for delivering th...