[thechat] Global lightbulbs

[Martin Burns]

On Thu, 27 Mar 2003, Tony Crockford wrote:

>
> > What would happen if you took a light bulb from North America
> > and screwed it
> > into a British fixture and turned it on?
>
> seriously?
>
> If you could actually screw it in (and I guess if you had an edison
> screw type fitting instead of the more usual UK bayonet type you could)

fwiw, most Ikea lamps have screw fittings, so finding lamps with screw
fittings isn't as hard as you'd think.

> Power = Current x Voltage (approximate in AC situation)

Accurate, rather than approximate, as long as you remember that the
voltage you're talking about is the Root Mean Square (RMS) voltage, not
the peak voltage.

If you visualise the AC voltage/time graphs as a sine curve, and average
out the voltage to RMS, you'll discover that the RMS value is maybe 2/3 to
3/4 the peak voltage.

RMS is a sleight of hand that averages out the varying voltage as if it's
a steady potential difference, which then makes all your standard
equations for electrics work out correctly.

The 240v and 11v we're talking about are both RMS values. Essentially,
ignore the peak voltage, and use RMS, and you're sorted.

> so a 100w US bulb plugged into a 240v supply would draw 1.96 amps, not
> enough to blow the fuse, but enough to make the bulb shine with 470w of
> power.
>
> So your US bulb would glow about 4 times as brightly, in a UK holder and
> would probably get very hot before burning out.

...unless it's a halogen bulb. Halogen bulbs are filled with a halogen gas
which enables bulbs to run much hotter (and therefore brighter) without
burning out. IIRC, the halogen helps the vapourised tungsten to reform on
the element, rather than on the glass of the bulb.

OTOH, this means that halogen bulbs get *very* hot, which is why many of
them have that odd silvery-backing that lets a bit of light out the back.
Actually, it's a dichroic filter that lets most of the low-frequency
radiation (ie heat) escape backwards and most of the high-frequency
radiation (ie visible light) go forwards.

That way, the thing you're lighting doesn't get stupidly hot - which is
why they're often used in exhibitions & museums, as you don't want your
objects getting hot, but you do want them to get lots of light on them for
the punters to view them.

Cheers
Martin

-- 
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