The coherer was indeed used more than 100 years ago not only by Marconi but also by Hülsmeyer who demonstrated the first operational radar (range about 3 km) in Cologne 1904 (he called it at this time telemobiloscope) and there was last year in November an IEEE milestone celebration at this very spot at the Rhine banks near the cathedral in Cologne . However the coherer required a regular gentle tapping on tis mixture of loose metal grains otherwise they would stick together (cohere).Essentially a kind of defined bad contact .Like in a normal RF diode the RF detection mechanism is based on the non- linear (in a certain region approximately quadratic) characteristic of this element. As for the theoretical explanations one has to cross check..maybe you are right.. maybe not. It might be good to look in the original paper by Walter Schottky (Around 1918) [in german] .

I can understand that we need a strong nonlinearity in order to get a Rf demodulation (like a normal diode) but I do not see the need for tunneling effects.They may be there as well but I would say are not required.

Hello Fritz,

Thank you for mentioning Christian Hülsmeyer and his radar system for detecting ships and the fact that his radar receiver utilized coherers. Too many people think that Robert Watson-Watt was the Father of Radar due to his tireless self-promotion, despite the fact that Arnold F. Wilkins, who had worked for Watson-Watt, wrote a review article on radar in 1953 that mentioned Hülsmeyer's priority in the matter.

A.F. Wilkins; The story of radar; Research Science and its Applicability in Industry; Vol. 6; No. 11; November 1953; pp. 434-440.

The first person to mention DC coherer behavior was P. S. Munck in 1835. Munck's name is misspelled in most literature as 'Munk'.

P.S. Munck; Versuche über die Fähigkeit starrer Körper zur Leitung der Elektrizität [Experiments on the Ability of Solid Bodies to Conduct Electricity]; Annalen der Physik und Chemie; Vol. 34 (2nd Series); 1835; pp. 437-463.

Idem; Untersuchungen über Elektriztät, mit besonderer Rücksicht auf die Theorie der galvanischen Kette [Investigation of Electricity, with special Consideration of the Theory of the Galvanic Series]; Annalen der Physik und Chemie; Vol. 43 (2nd Series); 1838; pp. 193-227.

The first person to use a coherer detector in wireless transmissions was David Hughes around 1879 to 1886. Like Hülsmeyer, Hughes' contributions were forgotten by more modern historians of technology. See the document titled "Coherers, a review" on my RG page for more details.

Unlike the modern diffuse junction semiconductor diode, the older point contact diode, and the electronic vacuum tube diode, which all have I-V (current-voltage) curves that go through the origin (I=0, V=0), the coherer in its uncohered state does not have an I-V curve that goes through the origin, that is, until it is cohered. Because its I-V curve goes through the origin, the semiconductor diode will conduct current - in either direction - even for very small applied voltage differences. In this I-V region around the origin, the semiconductor diode conducts current just like a resistor (ohmic or non-ohmic), i.e., there is no asymmetrical behavior otherwise the diode could be made to rectify its own noise current and in so doing violate the Second Law of thermodynamics - essentially creating a perpetual motion machine. Obviously, as you increase the applied voltage differences and get past the knee of the characteristic curve, around 0.7 VDC for a silicon diode, in the forward direction, you see the asymmetrical behavior.

My question has to do with the mechanism by which the coherer coheres, i.e., once it coherers, the coherer's I-V does go through the origin like any ohmic or non-ohmic resistor. Note, asymmetric I-V behavior is not required to detect, with a coherer, the CW transmissions by which Morse code is sent wirelessly, unlike in the case of AM modulation where the asymmetrical I-V characteristic curve is needed to create an envelope detector. To reiterate, my question has to do with how coherers cohere - the mechanism.

In the DC coherer effect, one simply increases the applied bias (DC voltage difference) until the oxide layer surrounding the metal filings breaksdown at the critical voltage difference. But, again, how does this breakdown occur? The AC coherer effect, which is what is utilized for wireless detection of CW modulation, is even more mysterious. Here, the DC bias is much less than the critical voltage, but the presence of a small amount of RF energy, from the receiving antenna, is enough to cause the coherer to cohere. What is the mechanism?

I hope the above explanation clears up what I was asking in my RG question.

You mentioned a 1918 article by Walter Schottky. Would you favor me with a citation? Was it the following citation:

W. Schottky; Über spontane Stromschwankungen in verschiedenen Elektrizitätsleitern [On spontaneous current fluctuations in different electrical conductors]; Annalen der Physik und Chemie; Vol. 57 (4th Series); 1918; pp. 541-567.

Again, thank you for your two replies.

Regards,

Tom Cuff

Hello Fritz,

One last point. Because semiconductor diodes or thermionic diodes have I-V curves that pass through the origin, they are always ready to detect modulation of the carrier frequency provided, of course, that the signal is of sufficient strength to rise above the noise floor. Coherers, on the other hand, require a two step process: they must first cohere, and then they can demodulate with the help of the mechanical tappers. I believe Marconi used two tappers one fast and the other slow.

The AC coherer effect is mysterious because the impressed RF signal potentiates the effect of the DC bias that is present across the coherer even though this DC bias is well below the critical voltage. Cere Davis , in a private message to me over RG, came up with the idea of using the word 'potentiate' and I believe it's an excellent description of the effect.

Regards,

Tom Cuff

Imperfect abstract translation from that paper (Fritz Caspers could probably do better as a native speaker):

By connecting cathode rectifiers in series, it has been possible to in recent years, generate alternating currents of

of extremely low amplitude and make them perceptible and measurable.

Some technical problems have been solved, but also the researcher seems to have opened up a new field.

The amplifier circuits have certainly have the same importance for electrical investigations as the microscope has in optics.

No clear limit has yet been shown for the amplification which has been shown so far, by sufficient protection

free mounting, etc., one could hope to penetrate here, so to speak, to the infinitely small; the dream of "growing grass" turned out to be true once again in a very tangible way.

The purpose of the following lines is to prove certain insurmountable limits for the effect of glow cathode and gas discharge tubes. The first insurmountable limit is given by the size of the elementary quantum of electricity. The warm movement of the

seems to continue to form a limit , in most cases to be higher than the other one in most cases. However

we send the investigation of this phenomenon as the simpler and better known one in front of our main consideration.

Article Über spontane Stromschwankungen in verschiedenen Elektrizitätsleitern

Hi Cere Davis ,

Re your last post. It is still not clear to me that the article,

[1] W. Schottky; Über spontane Stromschwankungen in verschiedenen Elektrizitätsleitern [On spontaneous current fluctuations in different electrical conductors]; Annalen der Physik und Chemie; Vol. 362 (Complete Series), Vol. 57 (4th Series); No. 23; 1918; pp. 541-567.

is the one referred to by Fritz Caspers in his first post, as he has never acknowledged it as being the one. Even if the aformentioned article is the one to which he was referring, it is not clear to me that this article is relevant to the discussion. [1] concerns itself mainly with noise in electron vacuum tubes due to the shot effect "Schroteffekt", see the section starting on p. 547 of the article, and see the abstract of the abstract of this paper found in Science Abstracts A; Vol. 22; 1919; p.128; Abstract #332 (see attached PDF); URLs: https://catalog.hathitrust.org/Record/002134014 , https://babel.hathitrust.org/cgi/pt?id=uc1.aa0010880292&view=1up&seq=7 , https://babel.hathitrust.org/cgi/pt?id=uc1.aa0010880292&view=1up&seq=138&q1=Schottky .

In any event, I still think Fritz needs to apprise us of whether [1] is the article, and - if so - how it explains the AC coherer effect.

Regards,

Tom Cuff

Schottky noise and nonlinearities are two separate issues.Even water and also human tissue have faint EM nonlinearities in the microwave range both of second order (good for rectification i.e. diode receiver) and also 3 rd order (as used for IP3 PIM tests.BAd contarcs are well known to produce a lot if PIM.,basically the working principle of the coherer used 1904 bv Hulsmeyer .Schottky was about 15 years later

Hello Fritz Caspers ,

Neither I nor Cere Davis has argued or suggested that shot noise and nonlinearities are the same. You were the one who, in your first post, mentioned that "It might be good to look in the original paper by Walter Schottky (Around 1918) [in german]." Of course, since you did not provide a full citation, we were not sure which paper it was. I suggested that the paper might be [1], but you have still not indicated if this is the paper to which you referred.

[1] W. Schottky; Über spontane Stromschwankungen in verschiedenen Elektrizitätsleitern [On spontaneous current fluctuations in different electrical conductors]; Annalen der Physik und Chemie; Vol. 362 (Complete Series), Vol. 57 (4th Series); No. 23; 1918; pp. 541-567.

If [1] is the paper, then I fail to see the relevance of this paper to the coherer question. If it is not the paper to which you referred, then, please, what is the citation?

Regards,

Tom Cuff