had formed in his early experiments two years
before--viz., that carbon had the requisite resistance to permit a very
simple conductor to accomplish the object if it could be used in the
form of a hair-like "filament," provided the filament itself could be
made sufficiently homogeneous. As we have already seen, he could not use
carbon successfully in his earlier experiments, for the strips of carbon
he then employed, although they were much larger than "filaments,"
would not stand, but were consumed in a few minutes under the imperfect
conditions then at his command.

Now, however, that he had found means for obtaining and maintaining high
vacua, Edison immediately went back to carbon, which from the first
he had conceived of as the ideal substance for a burner. His next step
proved conclusively the correctness of his old deductions. On October
21, 1879, after many patient trials, he carbonized a piece of cotton
sewing-thread bent into a loop or horseshoe form, and had it sealed
into a glass globe from which he exhausted the air until a vacuum up to
one-millionth of an atmosphere was produced. This lamp, when put on
the circuit, lighted up brightly to incandescence and maintained its
integrity for over forty hours, and lo! the practical incandescent lamp
was born. The impossible, so called, had been attained; subdivision
of the electric-light current was made practicable; the goal had
been reached; and one of the greatest inventions of the century
was completed. Up to this time Edison had spent over $40,000 in his
electric-light experiments, but the results far more than justified the
expenditure, for with this lamp he made the discovery that the FILAMENT
of carbon, under the conditions of high vacuum, was commercially
stable and would stand high temperatures without the disintegration and
oxidation that took place in all previous attempts that he knew of
for making an incandescent burner out of carbon. Besides, this lamp
possessed the characteristics of high resistance and small radiating
surface, permitting economy in the outlay for conductors, and requiring
only a small current for each unit of light--conditions that were
absolutely necessary of fulfilment in order to accomplish commercially
the subdivision of the electric-light current.

This slender, fragile, tenuous thread of brittle carbon, glowing
steadily and continuously with a soft light agreeable to the eyes,
was the tiny key that opened the door to a worl