A Tesla Coil Capable of 84-Inch Sparks

By
Ed Angell*

8476 Brier Drive
Los Angeles, CA 90046

This tale started one day when a client requested a device that would produce a spark long enough to run down a staircase. The effect was to be used in a horror movie film. I enlisted the aid of Dick Aurandt, the west coast electrical wizard. Another assistant equally adept at handling high voltages at high frequencies is Jim Shaffer. Jim is the kind of guy who can stuff 10 pounds of anything into a 5-pound bag. Other associates were also contacted for aid in the building and testing of the project.

During our initial meeting, we began plans for making a high frequency spark oscillator. Except for the power transformer, the project was to be totally self-contained. The capacitor bank and the rotary gap were designed so that they could be placed under the primary coil. Not long after, the ultimate idea emerged into reality as a large Tesla coil. One Saturday morning, we all assembled together to undertake initial tests in the tuning process.

When energizing a large Tesla coil, it is customary to start the tuning procedure at low powers. There is nothing worse than seeing a puny 10" spark coming off of a large coil project. Dick noted that the coil didn't seem to tune where the mathematics said it should. He had Victor, a math genius, rerun the numbers. “Vic” continued to do so until the batteries in the calculator died. In the meantime, assistant John Miller was wondering why the rotary gap motor was so warm.

After the best primary tuning was found, we opted to increase power. A 40" spark was achieved at an input of 6 KW. It was about this time when the tungsten contacts on the rotary gap decided to give up and die in a shower of white-hot sparks. John Miller's pocket thermometer was used to take the temperature of the gap motor. It went completely off the scale. In addition, it was possible to boil a few dozen lobster in the 20-gallon salt water rheostat we had devised as a power control. After a meeting of the council, it was agreed that the problem was created by having too many metallic parts in the proximity of the primary coil. The unit was behaving in similar fashion of an induction heater. Separating the capacitor bank and rotary gap from the coil proved what expert Ken Strickfaden (of Frankenstein fame) had told us long ago. “You have to keep all excess hardware away from the primary or it will change the inductance.” Boy, there's nothing like experience!

After adjusting the various primary leads and applying tuning techniques, we finally achieved our goal - 84" sparks! That's not bad when you consider that the secondary coil is only 50" long. The accompanying photos (next page) show the coil in its final arrangement. Photo #2 shows the spark gap mounted over the capacitor bank. The power transformer (not shown) is the typical electrical utility distribution type - 12,000 volts at 5.6 KVA. In some tests, we ran it for short intervals as high as 18 KVA!

Specifications are as follows:

1. Secondary------750 turns of #20 wire 14" diameter coil form (116 KHZ)
2. Primary--------3 turns of 7/8" copper tube
3. Capacitor------5 units @ .1 MFD connected in parallel

The coil achieved optimum performance with 2.3 primary turns and a .4 MFD capacitance. The experience gained from building this coil has been invaluable. The next coil I hope to investigate may be the diameter/height ratio of 2:1 of which Mr. Tesla was so fond. After all, he did achieve a spark of 135 feet!

* Ed Angell is a special effects agent for television and Hollywood movie studios. He appeared briefly in the Ripley “Believe It Or Not” program on Nikola Tesla.