Q. In Volume 14, #3, page 2, mention was made of Tesla obtaining a 12 inch spark with only 10 watts input. To take this into proper perspective, the number of breaks per second must be considered. For example, Breit, Tuve, and Dahl (Volume 14, #2, page 1) had a power magnification of about 500 at one pulse (spark) per second. My 350 KV coil uses 450 watts for an 11 inch spark. There are 120 sparks per second so the wattage is 450/120 = 3.75 watts. In other words, this coil produces an 11 inch spark with only 3.75 watts. Tesla's 10 watt coil would have to produce at least 3 sparks per second to equal this overall efficiency. My 70 KV coil uses 1.5 watts per spark for a 12 inch long spark at 80% efficiency?

A. Extrapolation is useful in determing hypothetical results. Your letter reminds me of a long-ago search for an Irving Berlin songsheet. I finally found one at an antique show but the vendor wanted $15. Berlin's music was printed by the millions so I thought the price a bit steep for a common title. The song was eventually found in a music anthology. The book cost me $12.95 plus $.91 tax and $4.50 packing and shipping. Since the book included 36 titles, the song that I wanted only cost $.51. What a bargain?

It would be interesting to know whether Tesla's claim was a matter of extrapolation or virtual reality. Perhaps the best method of resolving the question would be to weigh all of the units and determine the spark length per pound of equipment. Breit, Tuve, and Dahl would be the losers because their equipment was so heavy that it required a flat car to haul it. I don't know the weight of your 350 KV coil but I have a feeling that it is no where near as portable as Tesla's. It looks as though your 70 KV coil is the clear winner. But first, the judges would like to see a virtual reality photo of it putting out a 12" discharge on no more than 10 watts!

Q. A friend expressed an opinion that a line filter must be attached directly at the wall receptacle. He uses no other filters in his circuit?

A. I have no quarrel with using a power line filter at the wall receptacle. However, unless his system is operating within a Faraday cage (metal building, etc.), the power cord connecting the power transformer to the wall filter can become a source of RF interference. That is, it can be an RF transmitting antenna. Since few of us operate our systems inside of a Faraday cage, it's generally a good idea to place an RF filter at the input terminals of the power transformer.

Q. The Westinghouse neon sign transformer tables (Volume 14, #4, page 9) can be misleading. The tables show a large difference in volt-amperes for corrected and uncorrected transformers. However, input watts would be about the same for both types of transformers. The uncorrected transformers will use about twice the current of the corrected transformer. The current will be divided into active and reactive current and these currents are added as vectors. This will give an active current and input watts approximately the same as the corrected transformer. That's why neon sign transformer manufacturers publish tables that give capacity only in volt-amperes and not in watts. Neon sign transformers are just not designed for Tesla coils.

A. Your explanation certainly gives the tables a different look. However, I agree with Ed Kagle (and authors Miller and Samuels in Neon Sign Techniques & Handling) in stating “the higher the power factor, the closer the wattage approaches the volt-amperes.” I also agree with you regarding neon sign transformers for Tesla coil systems. Kagle brought attention to that point in his volume 12, #3 article.

Q. You have mentioned diathermy machines in several issues. I would like your advice in regard to one that I have recently purchased. The machine doesn't seem to perform (no heat from the pads, little activity from the ammeter) even though the tubes and components appear to be functioning properly?

A. Diathermy machines are simple, yet rugged, instruments made to last well beyond their time. I once found a unit in a junk yard. It looked like a basket case. Yet, I purchased it for $2 knowing full well that it contained components well worth more than the purchasing price. When I arrived home and plugged it in, the unit came to life and functioned as though it was new. Chances are good that there is nothing seriously wrong with your machine.

Assuming that the tubes are good, the only other components that usually go bad are the pads. A bad pad will have a broken connection where the conductor connects with the metal screen (inside the pad). You can test each pad by holding a fluorescent bulb just above each one when the unit is energized. If the pad is good, the lamp will glow brightly. A set of substitute pads can be made by connecting a conductor to a sheet of copper (also aluminum, metallic screen, etc.). Sandwich the sheet between two rubber pads slightly larger than the metal sheet. Glue or tape the perimeters together. A good pad will produce heat and show current draw at the ammeter. (I used 1/4" thick rubber typewriter pad to make my pads).