A 1.5 Million-Volt Tesla Coil

In defiance of countless FCC regulations, zoning ordinances, and common sense in general, I constructed this unusually-large Tesla coil in October 1988 for the Britannia Manor Halloween extravaganza in Austin, Texas. Sponsored by Richard Garriott ("Lord British" of Ultima fame), Britannia Manor was perhaps the largest and most ambitious spook house of its time.

Capable of delivering violent, writhing discharges of artificial lightning at levels well over one million volts, the Tesla coil saw extensive spook-house duty in 1988, 1990, 1991, 1992, and 1994. Here, we see the coil in operation at Austin's Discovery Hall, in preparation for service in their 1991 Halloween fund-raising event.

The Tesla coil's secondary was wound on a specially-constructed lathe with assistance from several Britannia Manor volunteers. The secondary coil was constructed in accordance with the specifications in Information Unlimited's BTC-5 plans, which called for 700 turns of high-grade #22 polythermalized magnet wire to be wound on a 20" (51 cm)-diameter cardboard form at a spacing of 10 turns per inch. This required almost a full mile of wire!

Pictured above are John Miles (the author, feeding wire to the rotating form), James Van Artsdalen and Ken Arnold (observing and critiquing the process), David "Dr. Cat" Shapiro (imparting the required angular motion to the form), and an unknown Britannia Manor victi-... er, volunteer applying one of several layers of shellac to the winding for moistureproofing.

By means of some fast talking and a $175 withdrawal from the treasury of Lord British, a 14,400-volt, 5,000-watt power-pole transformer was procured from the City of Austin's electric utility. This transformer, driving a rotary spark gap, provided excitation to the Tesla coil's primary resonant circuit. Normally used as a step-down transformer for power distribution purposes, the power-pole transformer was wired "backward" to step the 110- or 220-volt line voltage up to 14.4KV.

Initial tests on a 110-volt 25-amp circuit were not especially fruitful, as the Tesla coil would trip its circuit breaker after only a few seconds of operation unless several heavy-duty extension cords were placed in the primary circuit to limit the current. In subsequent years, the primary circuit was reconfigured for 220 volt operation with a large Variac serving as a rheostat. An unfortunate electrical fire in Britannia Manor's laundry room constrained the Tesla coil to 110-volt operation at reduced power during the 1988 Halloween event.

The 10-pole, 3000-RPM rotary spark gap was designed to optimize the coil's power output and overall menacing nature by generating several sparks per cycle of 60 Hz line current. Failure of the gap's motor due to high-voltage fields induced from the Tesla primary was a frequent annoyance, until additional RF filtering was installed in the motor's power circuit.

An overview of the complete Tesla primary circuit, showing the rotary gap and its auxiliary cooling fan. Also visible are the primary circuit's RF choke, intended to keep high-voltage standing waves out of the house wiring, and the primary circuit capacitor with its accompanying overvoltage discharge gap.

Although conservatively rated at 40 KV, the capacitor, not being an oil-filled type designed for high-energy pulse service, shuffled off this mortal coil after only a few hours of operation. It was replaced by a series-connected pair of 0.25 uF / 30KV oil-filled units from Plastic Capacitors, Inc., which proved refreshingly indestructible by comparison.

The Tesla primary winding consisted of 6 turns of #6 AWG solid copper wire, tuned to resonance with the secondary by means of an alligator clip (right) which could be moved up and down the coil. Mallory 8mm ignition wire was used throughout the primary circuit for its exceptional insulating properties. (The same brand of ignition wire later proved capable of conducting the megavolt-level secondary output several feet across the room without excessive leakage!)

Below are a few more demonstrations of the Tesla coil in action, photographed against the Texas Hill Country sky at varying shutter speeds and F-stop settings. Links to additional Tesla resources appear at the bottom of the page.

Lightning on Demand

Resonance Research Corporation

Bill Beaty's Tesla page -- a nice collection of links

Tesla's Invention Seen in Entertainment

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