Thirteen years ago, on VT, this observer predicted The New LEDs would eventually replace all light-bulbs – and give us giant TV screens. And as usual, he was right.
But this is not about that. It is about the good old incandescent light-bulbs – a few of which were claimed to have lasted FOREVER. Well, for a number of decades anyway.
We all read those local newspaper reports on bulbs which granddads had installed in sheds before WW1 – and which had only now gone phut.
But those pieces were written by hack journalists who had no idea what they were talking about. However, having long ago earned a sheaf of City & Guilds electrical qualifications, THIS writer DOES know.
And the fact is – those accounts of ever-lasting lightbulbs were BOGUS.
You see, the way a bulb works is this; you take a thin wire (the filament) – install it in an envelope of glass, in which all of the air has been replaced by a gas which does not support combustion (burning) – then run current through it, which causes it to glow brightly. Simple.
However, while the filament may not immediately burn out, it does EVENTUALLY die through degradation.
But the time that will take can vary ENORMOUSLY. And it totally depends upon the BRIGHTNESS of the bulb – which in turn, depends on the ratio of voltage to filament.
What this means is that the DELICATE BALANCE which enables it to glow brightly enough to light – but also allows it to LAST a reasonable time – is only a FEW VOLTS.
Which is how “double-life” bulbs worked – their filaments were merely LONGER, meaning they burned DIMMER.
And the WIRE between your ring-main and an appliance also makes a DIFFERENCE.
Just like the bulb’s filament, it has RESISTANCE – which warms it up. And this uses POWER – which DROPS THE VOLTAGE.
Now assuming a standard lead – only a few feet long and with wire of an adequate SIZE – the voltage drop along its length will be MINIMAL.
But what you will ALWAYS find is that these so-called “everlasting” light-bulbs were at the end of a LONG lead!
Typically, the afore-mentioned garden sheds, plus remote stock-rooms in builders’ merchants’ yards, basements, cellars and the like.
Thus, while their light output – with nothing else to compare it with – seemed quite normal, they were actually running UNDER-VOLTAGE for all those decades.
And in so doing, merely gave the APPEARANCE of being extraordinary.