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Dear
Ross,
You
know that if you light a candle, it gives off light. The problem
is that not much light is produced, considering the energy
expended. Thomas Edison discovered that if we pass electricity
through a thin wire, the wire can get very hot, so hot that
it actually glows. If we put the wire inside a bulb (so that
it won’t burn up), we have an “incandescent light
bulb,” which is our most common source of light. Again,
not much light is produced (but much better than a candle).
Now if we pass electricity through a glass tube with a special
coating, we get light, using less energy, and we have made
a “fluorescent lamp.” If we pass electricity through
certain gasses, we also get light, using even less energy.
For
street lighting, the gasses used are usually either sodium
or mercury. Sodium produces a yellow-orange light; mercury
a blue light. Such colors are OK for street lights, but not
OK for use in your house (if you used such lamps in your kitchen,
your mash potatoes would be orange or blue). For street lighting,
orange or blue is OK, especially orange (our eyes see orange
better than blue). So we put these lamps up on poles, and
point them onto the street, to make driving and walking safer
at night. Wires to carry electricity to the lamps are mounted
high on the poles, or inside the poles where you can’t
see them. The electricity is provided by the same source that
supplies your house: the local power company.
Many
years ago, lamps that burned whale oil, gas, or kerosene,
were used for street lighting. At dusk, a person called “the
lamplighter” would walk down the street with a long pole
with a torch on the end, and light the lamps. The next morning,
the lamplighter would put out the lamps. For electric lamps,
we need to block the flow of electricity with a device called
a switch. There is a switch that responds to light, called
a “photo cell.” If we point the switch at the sky,
it will allow the flow of electricity when it’s dark,
and block the flow when it’s light. That way, the street
lights will come on and off automatically, and we don’t
need a lamplighter!
Thanks
for your question,
Aubie and Dr. Gross
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Ross
Harrison