The Mighty 477 Blog: June 2007

Saturday, June 30, 2007

This was a group class project by the students in my 4th year apprenticeship class. Every year I gave Nikola Tesla a plug since I always found it unfair that the creator of every AC electric motor that we use today, plus all of the other inventions i.e. radio, radar, x-rays, blade-less turbines etc. went unnoticed while lesser inventors like Thomas Edison literally stole the spotlight. I was a little hesitant to build one since in the past it was difficult to find construction information, but it was the Internet to the rescue. After a lot of research and experimentation we came up with our final design. The primary coil was #8 copper ground wire wrapped around a plastic bucket, and the secondary was a six-inch PVC pipe with 1000 turns of #22 gauge wire. We used a neon sign transformer to feed the resonant circuit at 15,000 volts. I estimated the final voltage output at 750,000 volts at 50,000 Hertz.

In the photo you can see a student (right) holding a florescent lamp which is in no way connected to the circuit itself, but lighting because of the electricity flowing through the air, and another student (left) holding a copper rod drawing an arc from the antenna. Even though the charge was 750 kV, the shock was barely felt because the discharge was in the micro amps, and because the high frequency increased the "skin effect" which is the tendency for electrons to migrate closer to the surface of a conductor. The higher the frequency the greater the effect, so at 50,000 Hertz, most of the electrons were traveling over the surface of the person and not through him. All this knowledge still didn't comfort me when I was the first one to poke a rod next to the antenna.

Tesla's original concept was to build a large Coil to transmit electricity wirelessly though the air to receiving coils elsewhere across the earth. In Tesla's final design the Earth itself was the capacitor and the ionosphere was the return path for the current. All one needed was a receiving coil and you could have wireless power anywhere in the world you went. Cars wouldn't need batteries, your cordless drills would never need recharging, no more ugly transmission lines criss crossing the country, there was only one fly in the ointment: How do you charge people for the electricity. Tesla's dream of wireless transmission of power (and communications) ended when funding for the project was discontinued after the Wardencliff tower was almost completed.

Our first topload was a dryer hose placed on top of a board to hold it's shape. What looks like hair is sparks from the topload during a long exposure by the camera.

The Telsa coil needs a capacitor to store a charge and to form a resonant circuit with the primary coil in order to boost the frequency up (50,000 Hertz). After a failed attempt at using glass plates and aluminum foil, I discovered another "coiler" was using a salt water capacitor. The salt water, since it is conductive, acts like a plate in a capacitor, and the glass is the dielectric insulator between the two plates. One plate is in the bottle, and the other is the pool of saltwater outside the bottle. We used a copper pipe to bring the current to the capacitor. In later designs we used multiple smaller glass bottles because it increased the surface area of the capacitor thus making it larger. There are of course much more efficient designs, but we had limited time and money and it did serve it's purpose.

This shows the capacitor under operation. The light you see is spark discharge inside the bottle. This was probably not the most efficient operation since a lot of energy is being lost here, but it made an impressive display.

Antenna "Top load" made from two heavy duty pie tins and dryer hose with metallic tape over the surface. The Top Load has dual functions, it acts like a capacitor storing a charge between arcs, adding capacitance to help create a resonant circuit, and transmitting the electricity through the air to a receiver antenna.

Steve (Doc) Bernard putting the final touches on the primary coil which consisted of twenty turns of 1/4 copper tubing.

Secondary coil after the final coat of varish. The secondary consisted of 1000 turns of 22 gauge magnetic wire.

Close-up of Spark Gap

Electricians will recognize the parts used to make the spark gaps. Fender washers, 1/4-20 nuts, barrel nut and a Hilti concrete pin for the point. I used angle brackets to mount them on a board and to form a series circuit.

The spark gaps are arranged in series. Our original design only had a single spark gap, but after doing some research I found that multiple spark gaps increased the power of the coil, probably because the arc quenches faster. It also keeps the unit cooler since the arcs are spread over multiple points. Our original spark gap used to get red hot.