Building Tesla/Oudin coils & learning to think

September 13, 2012

Bernard Cole-September 13, 2012

How I built – and then rebuilt – a set of simple Tasla/Oudin coils for my ninth grade Science Fair project based on instructions from a 1915 DIY book of projects for young ‘boy mechanics,” and some of the unanticipated side effects of that experience.

Several of you who read my earlier DIY column wanted to know more about some of the simple electronics projects I built in my early pre-high school days, based on instructions I found in a book published 50 years earlier in about 1915 titled “Boy Mechanic: A book that tells how to build things.”

I do not have any record of the details of how I built the simple radio receiver I wrote about previously, but I did find the instructions for an earlier project that I took back to California from Oklahoma: making high frequency Oudin and Tesla coils with very mundane simple components, including cardboard tubes, magnetic wire, discarded phonograph records, and miscellaneous scraps of brass, wood and cloth fiber.

The reason I still have them is that upon entering ninth grade in high school, I used the same plans to build versions for the annual high school Science Fair, part of a trend that became popular in U.S. high schools in the 1960s.

Tesla and Oudin coils are both electrical resonant transformer circuits used to produce high-voltage, low-current, high frequency alternating-current electricity. The difference between the two is that the Oudin coil uses a lower current and is thus safer – both for the builder and the “zapee.”

The first versions built over a summer I spent on a farm in Oklahoma were, as noted earlier, were constructed with a collection of amazingly simple, and common place, components.

To make an Oudin coil (Figure 1 below) the instructions called for the use of a 6 by 11 inch cardboard tube for the secondary, which is first covered with two or three coats of shellac. When the last coat has dried, a single layer of No. 26 magnetic wire is wound on, with the winding started about half an inch from the upper end of the tube, one end fastened down to the tube and the other, about eight inches, left loose for connection to a brass rod.

Figure 1. Plans for an Oudin coil.

When the winding was finished, I gave it another coat of shellac, which was left to dry, while I created some of the other components. Several wooden disks - roughly the same diameter as the cardboard tubing - I made on my uncle’s lathe and fit into the ends of the cardboard tube at the center of the secondary coil.

The bottom disk was screwed into a wooden base and the top one drilled through the center to accommodate a brass rod leading to a brass ball attached at the top end. A cap for the coil was made from a seven inch vinyl phonograph record disk, with the hole in the center enlarged to make take the brass rod.

Small holes were also drilled at opposite points on the disk for small round head wooden screws which were used to fasten it to the wooden disk at one end. I found the brass ball I needed from a metal bedstead in a pile of “stuff” that my uncle had collected in the farm’s workshop.

The base of the Oudin coil was made of hard wood, supported and insulated from the ground by rubber crutch tips, which fitted over wooden pegs, one at each corner. After completing the secondary, it was necessary to build the coil’s primary winding, which consisted of eight turns of one inch copper ribbon attached to the base by four wooden cleats, slotted to separate the individual turns from each other. Flexible leads, with helix clips attached to one end were used to connect to binding posts to complete the coil.

Using a half kilowatt transformer and a single unit, oil immersed, high tension condenser, I was able to generate sparks 10 to 16 inches in length from the coil. In the first version, my test subjects were the many hens and roosters in the yard of the nearby chicken coop. What a commotion!!

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