City of Light: The Story of Fiber Optics

30 CITY OF LIGHT
sington district of London. His passion was building sensitive scientific instruments;
the classes he had to teach were unwelcome interruptions. Writer
H. G. Wells, who suffered through them, called Boys ‘‘one of the worst teachers
who has ever turned his back on a restive audience, messed about with
the blackboard, galloped through an hour of talk, and bolted back to the
apparatus in his private room.’’ 10
In 1887, Boys was on the verge of a measurement breakthrough, the start
of a few brilliant years that would make him a giant of British physics. He
wanted to measure the effects of delicate forces on objects. He knew that one
way to sense weak forces was to hang an object from a thread. The problem
was that the thread had to be thin, strong, and elastic to measure the forces.
Silk and spider line were the best fibers of the time, but they were not good
enough. Nor were metal wires, because they stayed bent if twisted too much.
Boys tested spun glass fibers a thousandth of an inch (0.025 millimeter) thick
but found them wanting because they stayed bent after being twisted, making
accurate measurements impossible. 11
After he had disposed of his classes, Boys retreated to his laboratory to try
drawing glass into strong, elastic fibers. He wanted them long, thin, and
uniform, so he wanted to pull the molten glass very quickly along a straight
path. He built a miniature crossbow, and made light arrows made by fastening
a needle to a piece of straw a few inches long. He stuck the arrow to one
end of a glass rod with sealing wax, and heated the glass until it softened.
Then he fired the arrow through two long rooms with a foot trigger. The bow
propelled the little arrow so forcefully that it could pull a fiber tail from a blob
of molten glass that hung briefly behind in mid-air before falling to the
ground. When the arrow landed, Boys found attached to it ‘‘a glass thread
90 feet long and 1/10,000 inch in diameter, so uniform that the diameter at
one end was only one sixth more than that at the other.’’ 12
The delighted physicist then tried his new toy on other materials. He had
started with ordinary glass, probably whatever lay readily at hand in his
laboratory, but like bread, glass can be made from many recipes. Strictly
speaking, glass is a solid inorganic material that never crystallized as it cooled
from molten form. Some materials form glasses when cooled quickly; many
others do not. The most important ingredient in common glasses is silicon
dioxide, a durable mineral known as silica. Nature can mold silica into clear
crystalline quartz, but we usually find it as sand.
You can make glass from pure silica, but not easily; it melts at about
1600�C (2900�F). Since antiquity, glass makers have added other compounds
including soda, lime, and ash to lower the melting point and give glass other
desirable properties. Glass specialists have developed particular recipes for
many purposes: fine glasses used in crystal ware, clear and uniform glasses
for precision optics, heat-resistant glasses for cookware, plate glass for windows,
and colored glasses for bottles. Silica is the key ingredient in all, but
glasses can be made in countless ways from innumerable mixtures of ingredients.