The Poulcen Arc: Exploring the First Electric Oscillator

[Quentief]

Hi everyone

I have read this article about the arc converter, also known as the Poulcen arc.

https://en.m.wikipedia.org/wiki/Arc_converter

It was apparently one of the first electric oscillators. Apparently, an electric arc was produced between two electrodes to put in resonance a RLC circuit, which makes an high frequency electric signal. I note this is exactly the same circuit that Mr Nikolas Tesla used to design his Tesla coil. However, it is claimed that the Poulcen Arc was able to be powered on 50 volts, but the question is how ? I mean, the air has a great breakdown voltage (3600 V/mm), so how an electric arc could be drawn with oy 50 volts ? Even those who build Tesla coils needs to increase the voltage at some kilovolt to power their setup.

Please, could you tell me what I misunderstood ?
Thanks in advance

 

[berkeman]

Thread closed briefly for review by the Mentors...

 

[berkeman]

Thread is reopened. Let's focus on the communication aspects and how the early ARRL enabled long-range communication. The inventors of those early radio communication systems were very innovative, IMO. Thanks.

 

[nsaspook]

The electric arc in that circuit functions as a form of Current controlled negative resistance for a RF frequency oscillator.

https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part5/page1.html

The tuned circuit stores energy supplied from the arc power source allowing for continuous RF on a single frequency at much higher usable transmitter power than the earlier spark-gap transmitters.

This transmitter used FSK modulation to transmit CW. The key 'on' would the remove the compensating inductance moving the frequency to the tuned range of the distant RF receiver and shift off frequency when the key was 'off'.

 

[tech99]

The arc oscillator, or singing arc, was invented by Duddell in 1900. It consisted of a carbon arc with a series LC circuit placed across it. It produced an undamped wave which could be modulated by a carbon microphone placed in series.

Notice this is fundamentally different to a spark transmitter, which produces damped wave trains. In order to form the arc, the electrodes are brought into contact then drawn apart. The later Poulsen arc gave high power, using a hydrocarbon atmosphere, such as ethanol or coal gas, with the arc working in a powerful magnetic field. Arc transmitters were typically used for low frequency operation but oscillation was reported up to 1 MHz.

High power arc transmitters of around 300kW at 24kHz were used from 1920 at Leafield, UK, for telegraph services to Egypt and the Empire. But when Rugby radio station was built in about 1925 it became possible to make valve transmitters of 500kW.

 

[sophiecentaur]

how an electric arc could be drawn with oy 50 volts

That's not an issue. Every DIY arc welder uses an arc which is struck, using initial contact between electrical trade and workpiece (17 to 40V). Cinema projectors used a carbon arc which used a feedback system to regulate the feed rate of one rod to control the arc current. I used a 16mm projector with an arc source. Very easy to use but needed a bit of skill to keep things set up properly.

 

[zoki85]

Hi everyone

I have read this article about the arc converter, also known as the Poulcen arc.

https://en.m.wikipedia.org/wiki/Arc_converter

It was apparently one of the first electric oscillators. Apparently, an electric arc was produced between two electrodes to put in resonance a RLC circuit, which makes an high frequency electric signal. I note this is exactly the same circuit that Mr Nikolas Tesla used to design his Tesla coil. However, it is claimed that the Poulcen Arc was able to be powered on 50 volts, but the question is how ? I mean, the air has a great breakdown voltage (3600 V/mm), so how an electric arc could be drawn with oy 50 volts ? Even those who build Tesla coils needs to increase the voltage at some kilovolt to power their setup.

Please, could you tell me what I misunderstood ?
Thanks in advance

When you make "mechanical" contact between electrodes, letting the current flow through them this way, and then try to separate them, the current keeps flowing due to some inductance in the circuit. Therefore, no high voltages are necessary and as long as the voltage of the source is higher than arc voltage the arc will be sustained. And no, the primary circuit is not the same as Tesla coil primary circuit. In arc transmitter there should be present, as you can see in your drawing,certain resistance in a series with a spark gap, and DC source is a must. OTOH, the resistance in T. coil primary should be as small as possible and high AC voltages are usually used as a power source.

The arc oscillator, or singing arc, was invented by Duddell in 1900.

By Elihu Thomson in 1892. He even patented it! Tesla also knew about the effect but considered it not worth working on it

 

[sophiecentaur]

Using an arc to make an oscillator involves a Negative Resistance Component. (see this Wiki link) As I read this thread, I was reminded of the Gunn Diode oscillator, which can be used to generate mWs of microwave power, based on precisely the same Dynamic Negative Resistance idea. I found that link which mentions the Gunn device. There's nothing new under the Sun, as they say.

That Tesla fellow missed out on the oscillator. He was fond of too many flashy ideas that never worked properly. lol

 

[zoki85]

That Tesla fellow missed out on the oscillator. He was fond of too many flashy ideas that never worked properly. lol

It was missed by many including Tesla, Thomson and Duddell. "Singing arc" set up worked well up to 10-15 kHz, and only at moderate powers. Nobody knew what to do of it and how to increase frequency and power throughput at the time. Fesseden latter made progress with idea of using high DC voltage and forming arc under high pressure. Poulsen made a progress by using hydrogen in an arc chamber, watter cooled copper anode electrode and carbon cathode etc. I guess Tesla was focused on making nearly perfect high power high frequency switch. That's bloody difficult. And problem with ordinary air arc is that V-I characteristic is almost flat in a high current range.

 

[Baluncore]

However, it is claimed that the Poulcen Arc was able to be powered on 50 volts, but the question is how ?

The arc is initiated when a momentarty contact is broken and the circuit inductance generates a high voltage spark. V = L*di/dt.

The low voltage arc is then maintained through the cloud of ions generated by burning alcohol from the drip feed. A flame has a low breakdown voltage.

 

[tech99]

By Elihu Thomson in 1892. He even patented it! Tesla also knew about the effect but considered it not worth working on it

I can never manage to sort out patent issues but the Wiki article credits the singing arc to Duddell. I was wondering if you have the Thomson patent number? https://en.wikipedia.org/wiki/William_Duddell

 

[zoki85]

I can never manage to sort out patent issues but the Wiki article credits the singing arc to Duddell. I was wondering if you have the Thomson patent number? https://en.wikipedia.org/wiki/William_Duddell

US500630
Also there's is wiki article about history of development of the arc converter mentioning Thomson

 

[nsaspook]

 

[tech99]

I am not certain that communication to the Titanic on two or more simultaneous frequencies, using additional operators, would be technically possible. How would the necessary transmit - receive isolation, in the order of 100dB, be achieved?
The presenter also mentions the lack of a distinctive distress signal. However, CQD (Come Quickly Distress) was in regular use and SOS (no spaces) had just been introduced and both were used by Titanic.

 

[nsaspook]

I am not certain that communication to the Titanic on two or more simultaneous frequencies, using additional operators, would be technically possible. How would the necessary transmit - receive isolation, in the order of 100dB, be achieved?
The presenter also mentions the lack of a distinctive distress signal. However, CQD (Come Quickly Distress) was in regular use and SOS (no spaces) had just been introduced and both were used by Titanic.

I agree, the Titanic tie-in was weak. I don't know the state of the art during that time but generally we would use a 20% separation in frequencies for 1970's era HF simultaneous frequency operations using passive tuned filters and separation S/R antennas for transmit - receive isolation.

 

[zoki85]

Again it turns out that greedy Marconi was a bad guy. Spark gap transmitters with damped waves were inferior to Poulsen arc transmitters in many respects off course. However I don't know if they were more reliable at the time.

 

[tech99]

Again it turns out that greedy Marconi was a bad guy. Spark gap transmitters with damped waves were inferior to Poulsen arc transmitters in many respects off course. However I don't know if they were more reliable at the time.

The Poulsen Arc is too powerful and difficult for maritime use, where the ordinary power was 0.5 kW. It also radiates CW, and receivers for CW did not exist at that time. It would have been necessary to use something like an interrupter wheel to obtain ICW - not a good solution.

 

[sophiecentaur]

In any case, they would all be breaking the law if they did that stuff these days. Bloomin' Radio Frequency Spectrum Hooligans!

 

[zoki85]

The Poulsen Arc is too powerful and difficult for maritime use, where the ordinary power was 0.5 kW. It also radiates CW, and receivers for CW did not exist at that time. It would have been necessary to use something like an interrupter wheel to obtain ICW - not a good solution.

Not sure about lower limits there but I guess that 500-1000 W Poulsen arc transmitter unit should still work. CW receivers for the purposes of telegraphy used a detector called "thicker" at that time. As concerns key signalling from the transmitter, in principle, it could be also achieved by shunt circuiting in the generator circuit or in the antenna circuit, by changing magnetic field in the arc gap or even by changing arc lenght. Interestingly that a crude radiotelephony technology was already existent at the time, but maybe considered unnecessary luxury on the ships since proven Marconi's technology worked good enough for basic needs

 

[tech99]

In any case, they would all be breaking the law if they did that stuff these days. Bloomin' Radio Frequency Spectrum Hooligans!

Maybe hooligan, but remember that Marconi patented the concept of tuning (patent 7777), so was personally responsible for the concept of spectrum sharing as applied to radio, and creating a concept that administrators would, by about 1990, understand and tax!
In 1907 Marconi patented a multiple tuner, which was a receiver front end filter having three tuned circuits. Even today, this would be considered good, and it was used on Titanic in 1912.
The detector used by Marconi was the magnetic detector, a concept Rutherford played with, and a ticker receiver was much less sensitive.

 

[zoki85]

Regarding the reception sensitivity, it's funny that the triode tube, the audion , was invented in 1906. But it took 6 more years, according to the wiki article, that its' amplifying potential for radio circuits was recognized.

 

[tech99]

Regarding the reception sensitivity, it's funny that the triode tube, the audion , was invented in 1906. But it took 6 more years, according to the wiki article, that its' amplifying potential for radio circuits was recognized.

In about 1912, three people, Armstrong, Meissner and Round seem to have simultaneously tried positive feedback with the triode detector, creating the very sensitive regenerative detector. This had a gain of maybe 1000 and so revolutionised simple receivers for twenty years. By increasing the feedback, the receiver was held just below the threshold of oscillation, where it was very sensitive. By increasing feedback, oscillation would occur and excellent detection of CW signals was possible.
Armstrong also invented the super regenerative detector (a different beast) during WW1. This had enormous gain, simple adjustment, broad bandwidth, AGC-like action and impulse noise suppression and proved useful for VHF/UHF receivers up to about WW2.