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Radio interference from Edison bulbs – Good Article Alert

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If there’d been VHF in the 19-teens, there would have been a big radio interference problem – from light bulbs.

We know this because we’re getting RF noise from some of the new vintage-style bulbs that are in production these days. There’s evidence that Rustika brand ones, for example, are accidental Barkhausen-Kurz oscillators.

So says Dr David Lauder, in an absolutely fascinating article in this month’s RadCom,1 the publication of the Radio Society of Great Britain.

Modern bulbs – the ordinary kind – don’t do this. But the Edison ones, the only ones in use before 1913, did. Their tungsten filaments emitted electrons thermionically, as from a cathode; some of the supporting wires inside the bulb could function as anodes. Presto! You had a houseful of vacuum tube diodes! ‘If the bulb was powered by DC mains,’ explains Dr Lauder, ‘the oscillation would be a coherent carrier’.

Sure enough, this is what some of the modern retro bulbs do, and they generate what amounts to wideband FM with sidebands smack in the middle of aviation radio, right next to the 2-meter amateur band.

Apparently, Edison bulb interference did become a problem eventually, that endured into the 1950s, when the last of the bulbs burned out. Typically, it was television interference between 41 and 68 mHz.




1 August 2020, Volume 96, Number 8, pp. 74-5.

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Good Book Alert: “The Radio Boys and Girls,” by Mike Adams

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Fans of Irving Vermilya’s backyard telegraph community (see my old posts about him), or the Hardy Boys and their shortwave mystery, will want to read this lovely book I’ve just found, The Radio Boys and Girls: Radio, Telegraph, Telephone and Wireless Adventures for Juvenile Readers, 1890-1945 (McFarland Publishing), compiling and discussing over 50 communications-related stories for young people from World War Two back to the 1890s.  It’s an obvious cultural history project, that no one has done until now — and I’m so glad Mike Adams has!  He’s got a website about it here, and Ed Sharpe, of the Southwest Museum of Engineering Communications and Computation, has a fine review of the book, here.

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For the scholars among us

Just to remind everyone, please help yourself to the growing Research Bibliography — look for it in the stack of links to your upper right.  It’s primary source-heavy, and it’s mostly about early wireless.  But it’s developing.  You’re welcome to tell me what more to put in it, too.  Drop me a line, and in it goes, with my thanks.  Meanwhile, browse it and use it!

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Technical Notes from 1908

WSB 1908

Electrician and Mechanic reader ‘W.S.B.’ wrote in from Brooklyn in the Autumn of 1908, with the picture above, to tell about his transmitting apparatus. He said this:

“Height of aerial to pole from spark-gap, about 25 feet, and the pole is 35 feet high, with umbrella aerial radiating from top of pole (eight ribs of 7-22 copper wire), each rib about 30 feet long, or total length of, say, 90 feet. Induction coil will give a 7½ inch spark. I use six glass plates, 10 X 12, with tin foil 7 X 9 inches on each side, connected in multiple, for my sending condenser, as shown by illustration. Battery, nine storage cells, worked at about 7 or 8 amperes. Have experimented quite a lot with independent interrupters, and I have now got one that will interrupt properly at a fast rate, and under current as above, without the points welding.”

This description referred to some concepts unfamiliar to me, though I’d heard the vocabulary. I went with questions to Roger Horton, K8CIX, contemporary builder of spark stations, who learned the art from his father and uncle, who started in the radio construction business in 1919.  See his magnificent site, Roger very generously filled me in on technical details I didn’t understand. My questions, and his answers, are below. Anyone who wants to know how spark radio worked should read this. Here are my queries, and here is what he said.

  1. Were antennas in 1908 pretty improvised? His sounds like an RF mess, designed just to get as much metal into the air as possible.

Well, it was early in the science and skin effect was known, so many experimenters did just as you describe and got as much wire up as possible, disregarding frequency or resonance.

  1. Is the induction coil the same as a tank circuit? Is its function just to build a voltage gradient big enough to arc?

No, the Helix or Helix Coil was actually a tank circuit that helped to determine the frequency of the emitted signal. It really had no use in the ARC performance. Really just a coupling device to couple the transmitter to the aerial. The induction coil was the coil at the beginning of the circuit that actually had a high turns ratio between the primary and secondary. Supplied with pulsed D.C. these coils usually developed between 14,000 and 20,000 volts which was then supplied to the Spark Gap.

  1. What is the condenser for? Is it a capacitor that just smooths out current?

Adding the condenser in spark gap technology was relatively simple. It entailed adding a capacitor across the secondary winding of the induction coil (or the spark gap) used to generate the spark. The addition of this single capacitor to the spark gap transmitter made a big difference. It eliminated the continuous arc which dragged down the voltage from the induction coil. Placing the capacitor across the secondary of the induction coil, in the transmitter, enabled both the gap current and the resulting antenna current to increase, and also the fast discharge of the capacitor removed the gap resistance from the antenna circuit. Both of these attributes come as a result of the addition of the single capacitor of approximately .05uF. The time it took the capacitor to charge kept the arc from occurring and at full charge the arc would then fire, also discharging the condenser and the sequence would start over, all occurring in milliseconds. This was called the oscillation or oscillator circuit. 

  1. What exactly is an interrupter?

There were 2 items referred to as interrupters. One, the device causing magnetic switching of the D.C. voltage supplied to the induction coil. This magnetically operated switch, called a buzzer or interrupter, usually was mounted on the front of the induction coil and the primary core of the coil caused the switch to interrupt the D.C. supply to the coil. The other type of interrupter was used in the ARC circuit to quench the spark gap and was called a Rotary Spark Gap.

  1. Why did the D.C. input current need to be pulsed? And I would have thought anyway that for a transformer, the current would have needed to be alternating.

Many of the places a spark gap transmitter was used had no access to A.C., but pulsed DC will operate an induction coil in place of the normal AC required to operate a transformer. Also, most of the early induction coils were designed for gas engines, such as autos, tractors, aircraft and hit & miss engines, and they ran on 6 vdc. 

If you remember back, just a few years, the automotive ignition system consisted of an induction coil, a set of points (interrupters), and a condenser, to create the spark or arc for the spark plugs.  

  1. And is the induction coil the same as a ‘loose coupler’, or is that for receiving?

Loose couplers are strictly for receiving using a crystal detector.


I am indebted to Roger Horton!  This makes early radio come alive.


Text and picture:  Electrician and Mechanic, October, 1908, p. 179, available at

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Amateurs underfoot in 1907

ruhmkorffAmateurs by 1907 were numerous, and intrusive, and many of them backward technically. Electrical World took notice of a policeman’s son in Washington who jammed the Navy Yard and harassed professional operators. From New York Lee De Forest thundered that the lad typified ‘the ubiquitous amateur with his high-school Ruhmkorf coil’ (pictured), who was not only a vandal but a slovenly practitioner of outdated and disruptive arts. Roving detectives were needed, and robust legislation at the Federal level. Improper damping was causing chaos, he foamed. Communications must be limited to the sustained oscillation methods. “The day of the barbarous spark discharge is numbered,” he prophesied, “and the sooner it is classed with the filings coherer the better.”

Says who:  Lee De Forest, “Interference with Wireless Messages,” Electrical World (June 22, 1907) XLIX, 25: 1270.

Picture source, forum page, Model T Ford Club of America.  Heinrich Daniel Ruhmkorff was a German instrument maker, d. 1877, who commercialized at mid-century earlier iterations of the induction coil.  This model is probably a Ford variant.  Amateurs cannibalized these routinely for spark gap transmitters.  The resulting signal was a wideband splatter.

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