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When Technologies Collide! Radio Interference from Light Bulbs

No, I’m not going where you think I am—this blog is not about compact fluorescent lamps.  It’s actually about a much older technology and how it caused problems twenty five years after it became obsolete! 

 My interest started with a story.  My friend, Dan worked for decades as a two-way radio system design engineer.  Dan told me that years ago an early television repairman explained how he tracked down a certain kind of TV interference.  If a customer’s screen showed a particular pattern, he would tell the customer he was going out for a smoke break.  The repairman would walk up and down the block looking for a distinctive looking light bulb in porch fixtures.  Porch lights sometimes are not used much, and a few of these 30 year old bulbs survived in that service all the way into the 1950s.  If the antique bulb was replaced, the television interference disappeared. 

 I wasn’t sure what to think about that story and although I owned a number of early carbon and tungsten bulbs I wasn’t sure exactly how light bulbs could create radio interference.  I forgot about Dan’s story until I came across the following picture in a 1954 Beitman’s Television Servicing book I recently purchased.

 

I immediately scoured the text for some explanation but there was none.  I called Dan and asked him some more questions.  I also called one of my old college professors, Doctor Dave, who has a PHD in electrical engineering, is an avid radio amateur, and collects early radios as well.  Doctor Dave said he actually had seen the phenomenon demonstrated years ago.  Radio wave production is quite an interesting feat when you have no resonant capacitor-inductor circuit!

 I queried the antique television forum at antiqueradios.com and began to poke around on the internet for any information on this obscure and somewhat forgotten phenomenon.  Yes, I found some very interesting things indeed, but first let’s look at a little background of the offending technology—much of it taken from James Cox’s excellent history of General Electric’s contribution to electric lighting, A CENTURY OF LIGHT.

 Edison’s first incandescent lights used a carbon filament.  Flameless electric lights had huge advantages over kerosene lamps and gas lights.  Lights that use a flame burn up the oxygen in a room, heat the room uncomfortably in the summer, and blacken the ceiling with soot.  Most flickered as well.  The one advantage electricity didn’t have was cost effectiveness.  Carbon electric light bulbs sucked a good bit of electricity at a cost much higher than kerosene or gas. 

 Two carbon filament bulbs:

At the beginning of the twentieth century scientists all over the world worked to find a more efficient light—experimenting with various metalized filaments.  Around 1906 European scientists were able to make successful tungsten filament bulbs.  These were much more efficient, producing some 7.85 lumens of light per watt compared with 3.4 for carbon bulbs.  The tungsten was very brittle, so further improvements were needed before it could be placed into general use.  After years of work General Electric’s William Coolidge found how to make tungsten truly practical for a light bulb.  In 1910 GE began to produce a light bulb using Coolidge’s durable drawn tungsten filament 1/6 of the diameter of a human hair.  The bulb had an efficiency of 10 lumens per watt.  GE named the bulb after the Persian god of light, Mazda.  Technical books written in the 1910s indicated that the future of electricity as a lighting source was not a foregone conclusion until the introduction of the Mazda light bulb. 

 

Some 20 inches of the tiny tungsten filament was strung between supports inside a typical Mazda bulb, zigzagging in a shape not unlike a two inch high cage inside the glass. 

The bulbs were evacuated to a high vacuum so gas molecules didn’t pull heat away from the filament, decreasing its light output. 

A working tungsten bulb with the distinctive filament shape:

These types of bulbs held sway until lamp makers learned to coil that 20 inches of tungsten into a compact coil much like today’s incandescent light bulbs.  Another GE chemist, Irving Langmuir not only found how to wind those tiny, tight coils, but also discovered that they retain the heat from the filament so well that vacuum within the bulbs could be replaced with inert gas.

 But until then there were millions of light bulbs made with straight filaments arranged into a birdcage pattern sealed in a vacuum.  It didn’t occur to anyone at the time that the bulbs could create radio waves! 

 Let’s bounce forward to the 1950s when televisions found a place in more and more American homes.   A small number of those early tungsten bulbs were still in use.  They had been merrily creating radio interference for decades.  Radio expert Hans Hollmann noticed and explained the phenomenon back in 1935 (more about that later) but I don’t believe public was aware of the problem because the bulbs’ oscillating  frequency fell outside the AM broadcast band.   The bulbs oscillated in the band that by 1950 was increasingly being used for the new FM radios and television.  I don’t know when the old tungsten bulbs were first singled out as a problem, but an April 1953 Popular Science article on FCC efforts to eliminate TV interference mentioned them. 

Under the heading “Light Bulbs the Culprit” George Waltz writes:

Even electric-light bulbs can be the source of trouble.  Worcester, Mass. Had an epidemic of TV interference until the power company offered to replace old-style tungsten-filament lamps with the modern variety free of charge.  Some 150 of the old lamps were turned in and the complaints from TV viewers dropped to nil.

 The article featured the picture below:

 

Again, in February 1958 Popular Science mentioned the bulbs in “The Remedy for TV Interference.” 

In a real or imagined conversation between author Jack Foster and interference expert Cam Evans the following dialog takes place:

“Not so simple,” said Cam, taking a clear light bulb, obviously an old-timer, from a drawer.  “Not many of these straight tungsten-filament bulbs around, but just one of them sure can mess up a lot of TV reception.”

“What’s the story?” I asked.

“Well, a whole two-block area was complaining of wiggly lines across the picture.  We had to make a house-to-house search before we found the culprit—this bulb.  A woman forgot to turn it off in an attic.”

The old tungsten light bulb got further coverage in a chart, although the artist drew a picture of a pre-tungsten carbon filament bulb.  Readers still got the idea.

So how do these bulbs produce radio waves?  I found a real treat on the web.  Joe Sousa has written an excellent article on a European antique radio site, Radiomuseum.org about what is going on inside the bubs.  I will not take away from Mr. Sousa’s writing by quoting too much—his article is definitely worth a complete read by anyone who has enough interest to have read this blog so far. 

Mr. Sousa first links to Hans Hollmann’s 1935 description and analysis of the phenomenon.  He then adds his own description of what is happening inside the light bulb, noting that the effect is not unlike the action of Klystron radio frequency oscillator tubes.  Once bulb makers coiled the filament and replaced vacuum with  inert gases, light bulbs stopped oscillating.  Mr. Sousa then goes on to describe his own experiments using reproductions of these straight-filament tungsten bulbs (yes you can get working reproductions of certain antique bulbs).  He collects some impressive documentation of the radio frequency characteristics of these bulbs.  Then he does something quite amazing—he impresses an audio signal and modulates the radio carrier emanating from the light bulb—actually using the bulb as a radio transmitter!  That’s so cool!  Mr. Sousa uses a scanning radio receiver since the light bulb’s radio carrier drifts as it gets warmer.

Please go read the entire article—it’s great. 

http://www.radiomuseum.org/forum/rustika_lightbulb_fm_measurements.html

Out of curiosity I played around the FM band with two early tungsten bulbs, but couldn’t create any noticeable carrier—Mr. Sousa mentions in his article that not every bulb will resonate (plus I didn’t want to spend a lot of time stressing my 90 year old light bulbs).

Technology sometimes has strange side stories.  Many contain a bit of mystery as well as comedy.  The story of the resonating light bulbs was certainly an interesting one to follow.

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