Tesla Coils - 1970 to 1973

After constructing a Van DeGraff generator, my friends and I decided to build a Tesla Coil. From Reading Popular Science, I had the impressiong that Tesla Coils required vacuum-tube circuitry, but I stumbled upon a diagram for the original 19th-century circuit, using a spark-gap oscillator.

Our first coil was powered by an oil-furnace transformer, and didn't work well because we didn't know the primary circuit had to be tuned to resonate with the secondary. We also used very heavy wire and not enough turns. Mark kvernevik from Thief River Falls (seen on the right with me in the photo) clued us in about tuning.

The larger coil was built in the summer of 1971. The power source was a 10 kilovolt line transformer donated by the Rural Electric Co-Op. The capaciter bank contained leyden jars made with quart jars covered with aluminum foil and filled with salt water. These rested on a copper sheet in a box, and wires hung from an over-head rail into the salt water. Remarkably, only one of these jars ever exploded. The primary circuit ran through 000-gauge aluminum wire.
We experimented with a number of quenched spark gaps: rotary, series and air-quenched. The final version was contained in an asbestos-lined box (for sound-proofing) and was air-quenched by a vacuum cleaner blower (silver box in photo). The oscillator was so powerful that it could boil a glass of water placed within the primary coil. The secondary coil pictured to the right produced about an 18-inch arc.
We started out being very ignorant about Tesla coils, but by 1973, I was an expert in pre-vacuum-tube electronics. We talked about building a Poulson Arc Oscillator, tone wheels, coherers. We also learned a great deal about the work of Nikola Tesla around the turn of the century.

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