Lightbulbs

[MWB]

I've heard that if you left a common lightbulb going it would never blow. They only blow when you switch them on and off. ??????

[heathfield]

The tungsten filament gradually erodes, and when it becomes thin enough it will melt at some point along its length. So it won't last forever, alas.
When you switch on, there's a very quick surge in the current passing through the filament. This causes the temperature of the tungsten to rise for an instant above its normal operating temperature. The result is that the weak point melts a short time before it would have done if the lamp were continuously lit. That's why bulbs fail more often when you switch them on, rather than when they're lit.

[RATTER15]

But they do also blow whilst still on!!

[MWB]

Thanks heathfield.

I've also heard that there is filament that has been around for many years that does not erode and will last practicaly forever, but the companies won't use/sell it as it would make the buying of new bulbs redundant; therefore their business' would not make anymore money.