REMAKING HISTORY
By William Gurstelle
Humphry Davy and the Arc Light
“The Dazzling Splendor,” as Davy called it, was
a tricky beast to control. After the initial sparks
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Thomas Edison did not invent the first electric
light.* More than 70 years before Edison’s 1879
incandescent lamp patent, the English scientist
Humphry Davy developed a technique for producing controlled light from electricity.
Sir Humphry Davy (1778–1829) was one of the
giants of 19th-century science. A fellow of the
prestigious Royal Society, Davy is credited with
discovering, and first isolating, elemental sodium,
potassium, calcium, magnesium, boron, barium,
and strontium. A pioneer in electrochemistry, he
also developed the first medical use of nitrous oxide
and invented the miner’s safety lamp. The safety
lamp alone is directly responsible for saving
hundreds, if not thousands, of miners’ lives.
But it is his invention of the arc lamp for which we
remember him here. Davy’s artificial electric light
consisted of two carbon rods, made from wood
charcoal, connected to the terminals of an enormous
collection of voltaic cells. (In Davy’s day, thousands
of cells, similar to modern chemical batteries, had
to be wired together in series to produce the voltage
required to strike an arc between the carbon electrodes.) When Davy closed the switch connecting the
battery to the electrodes, electricity jumped between
the carbon tips. The result — a continuous, glaring,
lucent dot of white heat — was so bright that it was
dangerous to look at for more than a split second.
appeared between the electrode tips, Davy had to
separate the carbon electrodes slightly and carefully in order to sustain the continuous, bright arc
of electricity. Once that was accomplished, he found
the device could sustain the arc for long periods,
even as the carbon rods were consumed in the heat
of the process.
Davy’s arc lamp of 1807 was not economically
practical until the cost of producing a 50V-or-so
power supply became reasonable. This didn’t occur
until the mid-1870s, with the introduction and commercialization of the electrical dynamo. But as soon
as that happened, arc lights were everywhere, archetypically in searchlights, as well as in lighthouses,
streetlights, movie sets, and movie projectors.
While making an arc light isn’t terribly complicated,
the arc’s underlying physical processes are indeed
complex. Although normally a nonconductor, carbon
will conduct electricity in certain circumstances. The
graphite rods used in arc lights conduct electricity,
albeit grudgingly, if enough electrical potential is
applied. At high voltage levels, the rod tips become
white-hot, and carbon particles break away from the
main body of the rod. Within the resulting particulate
mist, small bits glow white with heat and jump across
the spark gap between electrodes. This produces the
incandescent arch of light known as an electric arc.
It took a lot of juice to run a searchlight. To maintain its arc, a 60-inch-diameter World War II vintage
carbon arc searchlight drew about 150 amps at 78
volts, which is roughly equivalent to a 12,000-watt
light bulb. A lot of power, yes, but it could light up
an airplane 5 miles away.
Perhaps the largest carbon arc lamp ever made
was the 80-inch-diameter monster searchlight built
at the turn of the 20th century by General Electric.
It lit the grounds of the 1904 St. Louis World’s Fair
with its billion-candlepower arc light.
This month’s Remaking History project shows how
to make a smaller version of the dazzling splendor
in your workshop without too much trouble.
Image from National Portrait Gallery, London
BRILLIANT
MISTAKES:
Humphry Davy,
chemist, inventor,
and philosopher:
“I have learned
more from my
failures than from
my successes.”
It seems that Edison may deserve credit for the first
electric light bulb, but even that is controversial.
166 Make: Volume 20
