Incandescence from a pencil lead

Here is an old video of a basic incandescent light source made from a pencil lead and DC voltage source:

I liked this project because it reminds me of Edison’s original lightbulb design, which also had a carbon filament (made from charred bamboo). Naturally, the carbon gets exceptionally hot (incandescence, by definition, is the process of heating up an object enough that it releases light), and so this method risks shattering the carbon rod in a process similar to what happened in my arc welding project.

Most modern incandescent lights house a tungsten filament within a vacuum glass envelope, which extends the lamp lifetime by preventing the filament from oxidizing (“burning out”) in the air. Many theatres use what are known as Tungsten-Halogen bulbs, in which a reactive gas is contained within the envelope. As the filament burns, it reacts with the gas in an extended process that ends up eventually returning elemental tungsten to the filament. Of course, not all of the tungsten is deposited on the filament, causing TH lamps to gradually develop a “mirror” coating of tungsten that impedes their luminosity. But while this process is far more efficient than Edison’s original vision for incandescence, the mechanics of the process remain essentially the same.

Physically, incandescence can be modelled by what is known as a “blackbody” radiator, a hypothetical object that reflects no light. If such an item is heated to a specific temperature, it will start to release light of a very specific, predictable color (known as its “color temperature”). While no objects are truly black bodies, the construction provides a useful guide for predicting the behavior of real-word systems. As the name implies, by examining the color temperature or object, scientists can infer the temperature—a process vital to the study of distant stars and galaxies.

The reason for the relationship between color and heat is simple: As bodies (black or otherwise) absorb more and more heat, their electrons begin to jump to higher energy levels, in a process similar to carrying a heavy ball up a set of stairs: If I am willing to put in a little work, I can move a weight up one step at a time. If I put in a little less effort than what is need to go up a full step, I’ll end up not moving the ball anywhere all; the ball can’t go up half a step. Likewise, only certain, discrete amounts of heat energy(known as quanta) are absorbed by the atoms within an object. As these are absorbed, the electrons move into progressively higher-energy orbits– higher steps. Eventually, something happens to knock them off their high horse, and they end up falling back down to their original orbits– just as eventually my cat nuzzles my ball and causes it to fall back down the stairs. In falling, the ball gets rid of all of its energy by clattering and damaging my tile floors; in the atom, the electrons release their excess energy by emitting light.

The color of light is closely related to how much energy it has, and so higher energy/temperature objects tend to release higher energy colors. Visually, this actually corresponds to cooler tones– red hot is less serious than, say, white-hot or blue hot. Thus when astronomers see a blue star, they can tell that it’s still young and hot, because its temperature and appearance are inextricably linked.

Of course, there are other ways to excite electrons and create light– fluorescent and LED lights make use of stimulation from electrical fields to move electrons up the stairs, often at far greater energy efficiency than incandescent lights. But many lighting designers still prefer incandescence, purely for the warm glow caused by its low color temperature.

Worth noting in the video is that the light tends to flash on and off at a steady rate. Instead of using a battery, I opted for a 60 Watt AC to DC converter. My model, which was bought from a portable refrigerator company, includes a special protective circuit that shuts off the device when the current gets too high. After waiting some time, the current resumes, only to find that the short is still present.


One thought on “Incandescence from a pencil lead

  1. Laser Microscopy in 20 Minutes « gammacephei

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