Q. Can Tesla coils be made to operate for long periods of time, say 5-10 minutes without doing harm or shortening their life?

A. The answer to the first part of your question is yes - but not without shortening their life, the factors which determine just how long a coil can run without breaking down is in the ability of its components to withstand the heat and transients being generated. For long term use, measures must be taken to keep the components cool. The use of massive materials in the spark gap, or special cooling devices, help to slow gap deterioration. Capacitors in oil, and whose voltage ratings are multiples higher than the circuit voltage, will outlast units whose ratings are much closer to circuit potentials. Primary and secondary turns can become overheated if the conducting wire is undersized for the power being applied. But even the best designed coils can, and do, eventually fail. And that was true for coils built by the great one - Nikola Tesla.

I was witness to an Ed wingate demonstration of a properly designed rotating gap system. After running his coil for a period of 4-5 minutes, the gap was still cool enough to touch. I was somewhat reluctant to believe this was possible until coaxed into placing a hand on the device. It was only lukewarm. This is remarkable considering that the circuit was running on 12-15 KVA and throwing discharges more than 15 FEET!

Q. What resistance factor do you consider most important in designing a Tesla coil - DC or RF?

A. When constructing my first coils, I had little knowledge of RF resistance. However, I was aware of the DC component so this is what I was concerned with when building a coil system. Then, I read about the five million volt Tesla coil built in 1929 by Breit, Tuve, & Dahl. The scientists were able to obtain an enormous resonant voltage rise from a coil small enough to be carried by a junior high school student. The secondary was slightly over 3" in diameter and close to 40" long. It was wound with #40 wire numbering some 7000 turns! If you will take a moment to figure it out, your calculations will show an enormous DC resistance in the secondary. How was it possible for a coil that small with such a high DC resistance to produce 5 million volts?

Had anyone asked me whether or not this coil would work, I would have thrown my arms overhead and yelled, NO! Considering the results obtained with so high a secondary resistance, I concluded that the DC resistance in the secondary was not something with which to be overly concerned (but I know of others who would disagree with me).

Q. I have constructed a quench gap but have no way of operating it at a frequency of 500 hz (as was done in the days of spark telegraphy). Why does a quench gap work better at that frequency?

A. It doesn't. The 500 hz (1000 wave train) frequency was adopted because it was easier for radiomen to read the signals. When running on 60 hz (120 wave train), the signals sounded like a bunch of hash. There is no reason to believe that the quench gap won't work nicely on 60 hz.

Q. I'm constructing a vacuum tube oscillator Tesla coil and am mystified as to whether or not I have critical coupling. Have you any tips for the inexperienced with little knowledge of testing?

A. Since I have only constructed two vacuum tube Tesla coil projects (wasn't impressed with the unlightning-like discharges), anything I have to say on the subject will amount to the inexperienced telling the inexperienced. If you have a quality volt ohmmeter (VOM) or a vacuum tube volt ohmmeter, you might try the following tip which was passed on to me years ago by an electrical engineer.

Remove the secondary from the primary and take a voltage reading across the primary turns. This is only for a reference point so it doesn't matter what you get on the meter. Now inactivate the circuit and replace the secondary. Repeat the voltage measurement. If you have critical coupling, the voltage reading should be exactly half.