# Stupid Light Bulbs

Drakkith
Staff Emeritus

Thank god.

Quick, Om! To the patent office!

OmCheeto
It would store and release energy at the mains frequency (60 hz here in the states), but that has no effect on the lightbulb except to increase the impedance compared to a non-coiled filament.

I was told (in Britain) that when a bulb burned out, it was the back-emf in the coil that was responsible. Have't tried to verify this.

Staff Emeritus
2021 Award

The original question was why, using some badly estimated numbers (one more than an order of magnitude off), and an equation with fourth powers in it, the power calculated is a factor of 3.5 off. That's been answered - use more accurate numbers and the power calculated gets much closer to the number on the box. Hasn't that been answered adequately?

So what are we talking about now?

Drakkith
Staff Emeritus
So what are we talking about now?

We're talking about why filaments are coiled and not straight lines and how that affects the operation of the light bulb. If you have a problem with something in the thread, please report it.

Staff Emeritus
2021 Award
We're talking about why filaments are coiled and not straight lines and how that affects the operation of the light bulb

Thank you.

Filaments are coiled to get the resistance up. To get the resistance where you want it, you have two options - lengthening the filament (which requires coiling to make it fit) or narrowing it. If you narrow it, you are producing more heat per unit of filament (however you choose to measure it) and shorten the bulb life.

jim mcnamara and Drakkith
Drakkith
Staff Emeritus
I see. So then if you want to dissipate 100 watts you have to balance length and thickness to get the resistance at the right amount and also keep the temperature from either being too low (which will make a dim, inefficient bulb) or too high (reducing filament life).

Also, do you know if my assumption is post #13 about using less surface area in the calculation is accurate at all?

Staff Emeritus
2021 Award
I said heat, not temperature. The thing that destroys bulbs is heat.

I am not sure how important the shadowing effect is. If you model a coil as a hollow tube, the light's dependence on the inner radius is only through the thickness of the tube and thus its resistivity.

Drakkith
Staff Emeritus
I said heat, not temperature. The thing that destroys bulbs is heat.

I don't know what you mean by this. Could you elaborate?

Paul Colby
Gold Member
I don't have a reference but I recall coiling a filament reduces failure due to thermal shock. Time lapse movies of lights turning on show considerable vibration due to sudden temperature changes.