Light bulbs! We all need ‘em. We all use ‘em, and we all love ‘em (as long as they work) but how do they work anyway?
What Do I Need To Know About Light Bulbs And How They Work?
Okay. Light bulbs (known as incandescent) are really quite simple, and simply brilliant. The bulb has two metal contacts at the bottom of the base where they get their power from. These touch the electrical circuit in the fitting attached to your mains electricity, or any number of batteries, if we’re talking flashlights. The electrical charge used to light the bulb travels through it from one contact to the other in a loop. After hitting one contact the current goes up a wire to a filament, which is held on a supporting glass mount in the light bulbs center, then travels across it, down another wire identical to the first, and on to the other contact.
A Fundamental Filament
This filament is central in importance to the light bulb as well as central in position. It is made out of tungsten, which is a metal with an extremely high melting point, and it certainly needs one. After the light bulb is switched on, the tungsten filament is heated to between an incredible 2,200 and 2,500 degrees Centigrade! As well as its’ own properties, to further stop it burning up; the glass bulb does not contain any oxygen, but instead holds an inert gas called argon or a mixture of argon and nitrogen for all regular bulbs or krypton/xenon instead of the argon for more expensive premium models. (What about halogen bulbs? We’ll get to them later).
The filament is also tremendously long and thin. For example; in a standard 60 watt bulb the tungsten wire is over six feet long, but at the same time, it is less than one-hundredth of an inch in diameter. So how does it fit into such a tiny space? It’s double coiled, that’s how. Wound up tight to produce a first coil, then this coil is re-wound again to make the smaller than an inch filament that can be seen inside the bulb.
So the electrical charge heats up the filament to produce the light. How? The electrons that make up the electricity current rocket along, slamming into the tungsten atoms and causing them to vibrate. This friction produces heat or thermal energy, which is captured and then released by the electrons in the form of photons (light). Most of these are unfortunately in the lower end of the spectrum (known as infrared) and are invisible to humans. But the hotter the filament, the higher wavelength visible photons are emitted which we can see, and the brighter the light bulbs.
Higher wattage light bulbs have longer filaments, so they produce more light from having more atoms to vibrate, and conversely, low watt bulbs have shorter coils so the light is dimmer.
In three-way light bulbs, there are two filaments present with one being larger than the other. When the setting is on low, then only the small filament is used, so the light is dim. If the setting is put to medium, then only the larger filament has current traveling through it, and the smaller one is cut off from the flow. When the setting is on high; both filaments are in use together and the light is therefore very bright. To control this there are three connections (for the three operating modes). One each for both filaments exclusive use, and a third to be shared. A complicated switch controls the delivery of current.
Nothing Lasts Forever
So the tungsten filament is under tremendous strain, and won’t last. As the light bulbs are used for more and more hours the vibration and white-hot temperatures begin to take their toll. Increasingly the atoms from the coil will shake so much they will start to lose contact with each other and begin splitting away from their brethren.
In old vacuum light bulbs, they would shoot off into the space inside the bulb until they hit the glass. But with the argon or krypton inside them, modern bulbs last longer as many tungsten atoms hit the gas atoms and bounce around randomly, hopefully to reattach themselves to the filament if they get near enough to do so. Krypton atoms have more mass than those of argon and get more hits, but krypton is much rarer, so you have to pay for the benefit.
Though these light bulbs last longer, sooner or later the filament begins to disintegrate as the tell-tale darkening of the glass bulb increases (being caused by the errant tungsten) and your bulb will blow.
Often it might make the sound, ‘plink’, a few times and start flickering first, only to settle down again as if teasing you, so you decide not to change it after all and put the replacement you’ve just rummaged around for back again. Until next time you turn the lights on, that is, when it decides to give out after all.
Despite your suspicions to the contrary, however, the bulb is not getting its’ revenge on you for using it when not absolutely needed, or acting out of spite against humanity in general. It really can’t help it. A weak old bulb is at correct operating temperature when turned on for a while, but can’t reach it uniformly all the way along the filament when first switched on. There is always a surge of electricity drawn by a light bulb being turned on because of there being less electrical resistance in the tungsten when it is cooler. This resistance increases as the bulb heats up, but the weak spots in the filament heat up quicker than the rest (they have less surface area due to the evaporated tungsten atoms) and the funneling effect will cause these weaker areas to melt or snap due to the increased vibration.
So most light bulbs may not last that long, but they are relatively cheap and very plentiful.
Hello to Halogen
A halogen bulb works differently. It still has the same tungsten filament inside it as do the others, but here chemistry is employed in addition to physics to prolong its’ working life.
Inside these light bulbs, there is a halogen gas (almost always iodine) present, mixed in with the argon or krypton. This new gas reacts with the vaporized tungsten that collects on the glass to form chemical compounds called metal halides. These then leave the inner surface of the bulb in a constant recycling process and return to near the filament where the increased heat breaks down the halide into its constituent parts. The tungsten molecules are now given to return to the filament, and the iodine molecules are free again to join up with any more ejected tungsten.
This is known as the halogen cycle. The reaction only works successfully on the glass itself though, rather than the bulb’s inner space, once the tungsten has condensed and will not take place if the glass is not hot enough. Therefore halogen light bulbs have to be smaller (which increases the heat); handmade of a special higher-grade glass known as ‘hard glass’ or of quartz to allow them not to break at this extra high temperature.
Halogen bulbs cost more, but may have a lifetime of up to triple a normal light bulb of the same wattage, and at the same time be anything up to a fifth more efficient at producing light.
Long Lifers and Energy Misers
Long life light bulbs certainly last a very long time, so it might be argued that halogen is a waste of money. This is absolutely not true. A lot of these ‘long lifers’ are actually quite inefficient.
To burn longer they burn cooler, and at lower temperatures; a smaller percentage of energy is given off as light rather than wasted heat by the tungsten.
All light bulbs waste energy by giving off infrared light so the ‘energy misers’ out there on the shelves waiting for your basket to pass by may be considered a worthwhile option. But don’t be too hasty in gathering them up either. For some of these are not as good value as the cheap and cheerful regular guys. It might be claimed that a 55 watt can replace a 60, or a 90 will do for a 100 watt, but this is not necessarily so. Many (not all) of these ‘misers’ are more mean and miserly with light than with energy, and although it may not be noticed, sometimes produce less light by such a percentage factor that actually causes them to use more watts of power for a given unit of light if you chase this down the comparison scale.
Light Bulbs Rule OK?
So that is how they work. Not encyclopedic maybe, but a brief tour of a subject that matters to all; the incandescent electric light bulb. Still going strong after more than 120 years. Fluorescent lighting and LED‘s (Light Emitting Diodes) with their ‘cold light’ technology may be pushing more and more at the margins of their rule, but traditional light bulbs are still kings in our culture today.