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February 2019
Geek Challenge! Answer and winner

Geek Challenge! Answer and winner

02 February 2019

In our September 2018 issue we set a little challenge by asking readers to identify what was shown in the diagram.

The Solution

The answer is that it’s the outline schematic of Marconi-Poulsen arc radio transmitter from the very early years of the 20th century. Since it’s just possible that some of our readers are not familiar with this type of apparatus, we’ll provide a few more details.

In the very earliest days of radio communication, spark transmitters were used. Essentially, these comprised a source of high voltage, a spark gap, a tuned circuit, an antenna (aerial) and a ground (earth) connection. They radiated what we would now consider to be radio frequency interference (RFI) – damped oscillations rich in harmonics. Not surprisingly, this led to all sorts of problems with mutual interference between stations. Also, spark transmitters could only be used for telegraphy – essentially the transmission of messages in Morse code.

The key requirement for overcoming these limitations was to devise a way of generating continuous waves; that is, continuous oscillations at radio frequencies. Today we would use vacuum tubes (valves) or transistors, but back then even vacuum tubes didn’t exist in a form that would allow their use in radio transmitters. However, one thing the engineers of the time did know about was the electric arc. Specifically they knew that an electric arc could exhibit negative resistance characteristics – that is, as the current in the arc increases, the voltage across it falls. They also knew that if a negative resistance element (the arc) was combined with a tuned circuit, oscillations could be produced. Connect an antenna and a ground, and your continuous-wave (CW) transmitter is good to go!

The Marconi-Poulsen arc transmitter was one of the many designs to make use of these ideas but, as usual, producing a practical device involved overcoming a few challenges. In particular, it was difficult to devise an arc transmitter that would generate high power at what were then considered to be high frequencies (less than 1 MHz, often a lot less). The solution proposed by Danish engineer Valdemar Poulsen in 1903 was for the arc to burn in a hydrogen-enriched atmosphere, which increased (or should that be decreased?) the negative resistance of the arc.

This explains the inclusion of the alcohol feed in the diagram. Incidentally, alternatives used were coal gas (the type of gas commonly used at the time for room and street lighting) and gasoline (petrol). I wonder which of us today would feel comfortable about introducing a little gasoline into an arc chamber? By the way, a contemporary description mentions that the vessel E, at the bottom left of the diagram, helps to increase efficiency and “to stop explosions”. Presumably the engineers of the time were very grateful for that small benefit! Sharpeyed readers will also note that the arc operates in a magnetic field, which also improved the characteristics of  the device.

The Marconi-Poulsen arc transmitter is just one of many similar devices that were developed around the same time, but all were ultimately displaced by the much more efficient and compact radio transmitters based on developments of Lee de Forest’s Audion or, as it is better know today, the triode vacuum tube.

The Entries

Our first Geek Challenge produced a large number of entries, but only three of these were entirely correct. Many of the others were ingenious but rather wide of the mark – for example, “a cloud chamber”, “a device for extracting energy from light”, “a diamond synthesis reactor”, and – a personal favourite – “a beer carbonator”! That would definitely be a new departure for  Electrical Tester! Some of the entries were much closer without being spot on. Several suggested that it was a spark-gap radio transmitter but, as explained earlier, the Poulsen Arc transmitter is actually a much more sophisticated device. A few of the entries simply said, “a radio transmitter” or “an oscillator”. Both of these answers are technically correct but, as we had entries that specifically named the Poulsen Arc transmitter, we decided that they were not specific enough to be contenders for the prize.

The person who submitted the first correct entry was James White from Norwich, UK, and, by the time this appears in print, will have received his prize of a £100 amazon voucher plus a box of Megger Goodies! The second runner up, who submitted the correct answer but later than our winner, was Mario Cannistra from Torino Italy,  who  received a selection of Megger goodies.

We’d like to thank all those who entered, whether or not they got the answer right, and we hope all of our readers will find our next Geek Challenge just as interesting. Look out for it in a future issue of Electrical Tester.