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An article in the May 1919 issue of the Electrical Experimenter, "The True Wireless," talks about a "rotating brush wireless detector" that Tesla used, and in the January 1919 issue there is a photo of a device identified as "Tesla's Static Eliminator."  Do you have any knowledge of these devices?

The "rotating brush wireless detector" was one of a broad array of experimental single terminal vacuum tubes designed to be operated in conjunction with one of Tesla's high-voltage power supplies. The construction of the tube itself was very similar to the now popular "plasma globe" novelty item. In one form it consisted of a glass globe in the neck of which was sealed a glass tube. The end of the tube was blown out to form a small sphere positioned at the center of the larger globe. The connection to the power supply was made through a moderately rarified, highly conducting gas enclosed within the central tube. You can learn more about this specialized vacuum tube, the rotating brush phenomenon, and other types of single electrode tubes in Tesla's lecture "Experiments with Alternate Currents of High Potential and High Frequency" which appears in the book Inventions Researches and Writings of Nikola Tesla.  The actual description, with illustrations, appears on pages 226-230.

Regarding Tesla's static eliminator, as you have discovered, there is relatively little info available about this dinner-plate size device. It was a variable coupling RF transformer consisting of two flat spiral or pancake coils.  The uppermost coil could be raised on a vertical central column and locked in place above the lower coil at the desired separation.  Tesla spoke about the static eliminator in a 1916 interview (see Nikola Tesla On His Work With Alternating Currents) saying,

   "This instrument was built [at] a recent date, but the principle involved in its operation I have employed in Colorado. It is what I call a static preventer. A great trouble, when I came to Colorado, was that I could not operate at all. You know that the static interference is today the great bane of the wireless transmission. The reason for that is defective construction of the plants, but with this invention I am enabled to even make these defective plants operate satisfactorily.
   "The principle involves the employment of a plurality of tuned circuits in series as shown [in Fig. 77]. However, any interaction of the circuits, arising from mutual induction, produces results which are exceedingly complex. The emitted note will not be pure because of the beats produced. Radio engineers are not fully appreciative of this. The instrument shown [Fig. 101] consists of coils suitably arranged in demonstration of the principle to overcome the detrimental effects."

By the way, both of the Electrical Experimenter articles can be found in the book Solutions to Tesla's Secrets.

If the static eliminator received a patent, it must have been issued by a country other than the United States.  Nevertheless, it is not included in the partial list of patents published in the CATALOGUE OF TESLA PATENTS, Belgrade, 1987.  Using data found in this list and other sources [1,2,3], it is apparent there were at least 236 patents issued to Tesla in 25 countries over the course of his lifetime. This figure breaks down as follows:


Number of Patents Listed

Actual Number Found
Argentina 1
Australia 14
Austria 4
Brazil 2
Canada 2 6 (+4)
Cuba 1
Denmark 1
Germany 14 19 (+5)
Great Britain 16 22 (+6)
France 13
India 1
Italy 11
Japan 1
Hungary 7
Mexico 1
New South Wales 2
New Zealand 1
Norway 2
Rhodesia 1
Russia 4
Spain 4
Switzerland 4
Sweden 2
Transvaal 1

United States

112 112 (=)
Total 221 265-350 ?

While there are definitely a number more patents than those listed above (see Canada, Germany, Great Britain and the U.S.), the often quoted figure of 700 patents appears to be excessive.  Judging from the available data, the actual figure probably lies somewhere around  325 Tesla patents worldwide.

[1] Dr. Nikola Tesla � Complete Patents 
[2] Nikola Tesla Patent Collection 
[3] Nikola Tesla � Collected German and American Patents 

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