Light Bulbs and the Electric Hum

Copyright © Karl Dahlke, 2023

Here's a memory from when I was 9, which still fascinates me today. I had a toy called Electronic Dominoes, a wonderful teaching tool. Kids can put circuits together, and there's an instruction booklet. If it is still on the market you should buy it. I arranged, in series, a battery, a photoconductor, and an earphone. The photoconductor conducts electricity if light is shining on it.

🌃 dark room, no current, no sound. That makes sense.

🌞 Sunshine, lots of current, but it's a steady current, with no vibrations, so still no sound. I was draining my battery for no purpose - until I realized what I was doing.

💡 Now try the lamp in my bed room, a traditional light bulb, the kind we used for 100 years. I could hear the hum, the low hum of residential electricity. Somehow that hum was carried across the room in the form of light. How is that possible?

A light bulb is the simplest circuit you will find. electricity runs through a short thin filament made of tungsten. This wire has high resistance, and gets hot, 5,000 degrees (f) hot. fortunately, tungsten doesn't melt, even at these temperatures, and it doesn't burn in the air because all the air is sucked out of the light bulb, leaving a vacuum. So it glows white hot, (almost white hot), and lights up the room.

Household electricity is alternating current, 60 cycles per second. The current goes back and forth 60 times a second. It runs left to right, for example, and the filament gets hot, then it slows down and stops and the filament cools, then it runs right to left, and the filament gets hot, then it stops and the filament cools, then the cycle begins again.

→ 🛑 ← 🛑 → 🛑 ← 🛑 → 🛑 ← 🛑

Current is forced through the filament 120 times a second, and electrical energy is converted into heat and light. In 1/240 of a second, the current goes from its maximum down to 0. I'll call that 4 milliseconds. Here's the surprising part. As the current drops from its maximum down to zero, the filament cools, in just 4 milliseconds. How can anything cool that fast? Your cup of coffee takes several minutes to cool. Well the filament is tiny and thin, and very hot, and things cool off faster when they are small and hot. I'm sure it doesn't cool all the way down to room temperature, there is some thermal inertia here, but it cools off enough that it puts out less light. Brighter, dimmer, brighter, dimmer, 120 times a second. If you could slow down time you would see the light bulb bright and dim, bright and dim, like a strobe. Of course 120 times a second is too fast for you to see. The tv screen refreshes 30 times a second and that's too fast to see. But with my little circuit, I could hear it. The light went brighter and dimmer 120 times a second, the current in my circuit changed 120 times a second, and I could hear the 120 hertz hum in my earphone, B below C below middle C, also known as B2.

I wonder how much the temperature of the filament actually changes in these few milliseconds. From 5,000 down to 4,000? That would be enough to make a sound in my ear piece; the human ear is very sensitive. It would be interesting to graph the temperature across the entire cycle - though I don't know how we would measure it. The filament is in a glass bulb in a vacuum. I suppose we could measure the emitted radiation with a spectroscopic high speed camera, and then use the black body equation to reverse it and infer the temperature, millisecond by millisecond.

Light Bulbs that use Other Technology

Would a fluorescent bulb, or an LED bulb, exhibit such a cycle? Sadly, I don't have my Electronic Dominoes any more, so I can't test it. I believe the answer is yes, with the strobe effect even more pronounced, because there is no thermal inertia to carry the light across the fluctuating current. When the current stops, the fluorescence stops, completely. There is nothing to "cool off". You can't see this high speed flicker at a conscious level, but long term exposure seems to cause headaches in some people. If transmuted into sound, the hum would be louder in my ear piece.

This flickering effect could be reduced by a high quality rectifier, to turn ac into near constant dc.

Transmitting Voice over Light

Since I had two full sets of Electronic Dominoes, I might have been able to perform the following experiment, though I never did. Let an ear piece serve as a microphone, convert sound into an amplified electrical signal, and run that through a tiny light bulb. Then, on a separate board, replicate the photoconductor circuit described above. I had enough pieces to do this. I could have talked into my microphone and heard my voice in the ear piece, after the sound travelled across the room in the form of light. This assumes all the other lights in my room are off, leaving only the small light bulb with my voice imprinted upon it. Other than scientific curiosity, why would anyone want to do this?


In 1927, the movie industry was interested in this technology, as they developed the first talkie, The Jazz Singer. Prior to this, movies were silent, just a moving picture, thus the name movie. Sometimes a live piano player would accompany the movie, just so there was some sound. The movie was recorded on film at 24 frames per second. If you looked at the tape you could see little pictures in a row, one after the other after the other. The following depiction represents 24 frames, one second of action. This much film runs through the projector every second.

■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■

The tape rolled past a light bulb, as a shutter snapped open and closed, projecting images onto a screen 24 times a second. This gave the illusion of motion, although once in a while it looked like a flicker, thus a movie was also called a flick.

“I saw a good flick yesterday.”

Now the company that offers movies over the internet is called Netflix.

But people wanted to hear the conversations, along with the images. We knew how to record sound on tape, even before the movie camera was invented, so what's the problem? The issue is synchronization. After ten minutes of movie, the sound and the images could be out of sync by several seconds. The voice tape could run just a hair faster or slower than the picture film. When lips and speech are out of sync, the result is awkward, to say the least. The only solution is to record the sound on the same film as the pictures. That means sound must be recorded on photographic film, in the form of light. After 1927, a movie consisted of little pictures in a row, 24 frames per second, as before, and a line below, a track that encoded the voices of the actors. Microscopic light and dark regions along this track corresponded to vibrations of sound. As the movie played, a tiny beam of light passed through this track and onto a photoconductor, which produced vibrations in an electrical circuit, and then sound in the speaker. 🔊 The film might look like this.

■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■

The path of light and dark patches below the pictures was called the sound track. It was a track of light that encoded the sound of the movie. To this day, the music or songs that accompany a movie are called the sound track.

Methods of Modulation

Encoding sound in this fashion is called amplitude modulation, or AM. The strength of the light changes, bright dark bright dark, in sync with the voices or songs. The amplitude of the light, another word for its strength, encodes the vibrations. Light is the carrier, and it modulates in strength, modulates to carry the sound. Yes, this AM has the same meaning as AM radio. More on that later.

What would frequency modulation, FM, look like when light is the carrier? This is impractical, but let's pretend. You need a special kind of light source that emits just one color, something like a laser. But it has to be a variable laser, producing all the colors in a narrow range. Light is still the carrier, it still carries the sound, but the laser changes color, from red to yellow and back to red again, passing through all the shades of orange along the way, in sync with the sound. The color shift encodes the vibrations. A detector on the other side of the room, or city, tracks the change in color and turns it into variations in an electrical circuit, and then to sounds in a speaker. Lasers don't work that way, they aren't like a slide trombone, thus this scheme is impractical, but FM radio is entirely practical, and desirable. More on that later.