Q. I looked up the OMNI (March) article on Tesla mentioned in TCBA NEWS, Volume 7, #3, and found it to be disappointing?

A. We Teslians tend to look forward to each article on Tesla with eager expectation that it will be the definitive version. We won't find it in OMNI. I tend to read articles on Tesla in terms of being FOR or AGAINST. Those I consider to be against are the technical papers that either ignore Tesla's contributions or attribute them to someone else. I did not consider the OMNI article to be against Tesla although I, too, had some critical opinions of it.

Q. I am a beginning Tesla coil builder and would like your opinion of the Tesla coil project featured in October issue of HANDS-ON ELECTRONICS?

A. I am puzzled as to why this project received cover attention. Perhaps it's because editors seem to be enamored over anything solid state. That is, the new technology is better than the old. This view may be true in some cases but not here. No one has built a better Tesla coil than the GREAT ONE. The technology may be old but it beats solid state designs when it comes to power handling and the production of high voltages. My suggestion would be to construct the Kendall Ford project as mentioned in one of the letters from a TCBA member. This coil will not only outperform the Rakes project but can also be updated to exceed the efficiency of the 1935 design. (See TESLA COILS RESURRECTED).

Q. I am constructing a project that calls for an isolation transformer. This item tends to be a bit expensive for my pocketbook. Do you have any suggestions?

A. My high school science teacher showed me how to overcome this problem way back in the late 1930's. Hook two filament transformers back to back. That is, the low voltage side of one is hooked up to the low voltage side of the other. The 115 volt terminals serve as both input from one and the output of the other transformer. It is possible to obtain two transformers with low voltage/high current secondaries for less than $20. You won't find an isolation transformer at that price. Check the catalogs of Herbach & Rademan, Meshna, and Fair Radio Sales (addresses given in several issues).

Q. When I energize my coil, I get a buzzing yellow flame at the spark gap. After a few seconds, the fuses blow. What is the cause of this and how can it be prevented?

A. The condition is known as arcing and is due to poor quenching at the spark gap. Heavy 60 Hz currents bridge the gap instead of the usual snappy high frequency blue-white discharges. You can correct the condition by employing a rotary gap or by placing an iron core (reactance) choke in series with the low voltage side of your transformer. Generally, high powered Tesla coils require both. Adjust the choke to the condition where the arc is choked off and the gap becomes bridged with the bright blue snappy discharges. Or, if this is not possible, substitute a high reactance (neon sign type) transformer in place of your present power supply.

Q. Can you explain the technical reason why chokes and safety gaps are required in the primary circuit? I know that they are used to protect the components but do not know the reason why the circuit needs protection?

A. A high frequency circuit develops a tremendous electrostatic field. This is due to the fact that both in-circuit conductors and other metallic components become something like plates of a capacitor. This electrostatic condition produces high potential currents that tend to run wild through the circuit. When the transients run back through the circuit and into your transformer, they see the transformer windings as chokes that oppose their travel. An analogy might be a great body of free-running water suddenly being stopped by a dam. If the dam doesn't break, then the forces must be diverted in other directions (such as over its river banks). The choking effect inside the transformer forces the currents to find a point of least resistance (such as turn-to-turn discharges). ZAP! You've just lost a transformer. A choke is put into the circuit to oppose the transients before they reach the transformer. The safety gap acts as a relief valve by diverting the energy to ground.

Q. What is self-capacitance and how does it effect a coil system?

A. It's a very complex topic and involves not only the effects of electromagnetic and electrostatic fields but also impedance, the dielectric of the coil form and wire insulation as well as frequency. Under certain conditions, a self-capacitance can have little or no effect while in some situations the self-capacitance can produce a power loss. The turns of a conductor can act as though they are small capacitances. Should an inductor resonate with the individual capacitances of the turns, there would be more than one resonant frequency.

Q. How can self-capacitance of a coil be reduced?

A. The self-capacitance of a coil is directly proportional to the length of the coil. Therefore, it would appear that shorter coils would have less self-capacitance than coils wound on long forms. The type of design that would have the smallest self-capacitance would be one wound in the form of a flat spiral. Tesla recognized this and constructed a number of successful coils in that form. Other ways to reduce self-capacitance is to keep the individual turns spaced or wind the coil in sections and connecting them in series.

Q. I have had absolutely no success in getting my coil running. The secondary is 10' tall and 14" in diameter. It is wound with #26 wire. The capacitor is made up of a number of gallon jugs. Can you suggest some ways to get this project working?

A. I've always had the notion that even a poor coil can be made to work. You've made me eat my words. The secondary form is too long, the wire too small, the capacitor inefficient. You didn't tell me about the primary coil, the spark gap, or the transformer supply so I can't help you there. Cut your secondary down to about 3 1/2-4'. Wind it with #18, 16, or 14 gauge wire. Number 16 would be a good compromise. Exchange the capacitor for a more efficient unit. Even a PCB cap designed for 60 Hz power circuits would be better than a bunch of jugs. I also recommend that you read through our earlier issues on PRINCIPLES OF TESLA COIL CONSTRUCTION. We covered the design and construction phases pretty thoroughly so that such errors would be avoided.