The Invention Of The Incandescent Lamp - 8

The Invention Of The Incandescent Lamp - 8

Continuing the digression one step farther in order to explain the term "multiple arc," it

may be stated that there are two principal systems of distributing electric current, one

termed "series," and the other "multiple arc." The two are illustrated, diagrammatically,

side by side, the arrows indicating flow of current. The series system, it will be seen,

presents one continuous path for the current. The current for the last lamp must pass

through the first and all the intermediate lamps. Hence, if any one light goes out, the

continuity of the path is broken, current cannot flow, and all the lamps are extinguished

unless a loop or by-path is provided. It is quite obvious that such a system would be

commercially impracticable where small units, similar to gas jets, were employed. On the

other hand, in the multiple-arc system, current may be considered as flowing in two

parallel conductors like the vertical sides of a ladder, the ends of which never come

together. Each lamp is placed in a separate circuit across these two conductors, like a

rung in the ladder, thus making a separate and independent path for the current in each

case. Hence, if a lamp goes out, only that individual subdivision, or ladder step, is

affected; just that one particular path for the current is interrupted, but none of the other

lamps is interfered with. They remain lighted, each one independent of the other. The

reader will quite readily understand, therefore, that a multiple-arc system is the only one

practically commercial where electric light is to be used in small units like those of gas or

oil.

Such was the nature of the problem that confronted Edison at the outset. There was

nothing in the whole world that in any way approximated a solution, although the most

brilliant minds in the electrical art had been assiduously working on the subject for a

quarter of a century preceding. As already seen, he came early to the conclusion that the

only solution lay in the use of a lamp of high resistance and small radiating surface, and,

with characteristic fervor and energy, he attacked the problem from this standpoint,

having absolute faith in a successful outcome. The mere fact that even with the successful

production of the electric lamp the assault on the complete problem of commercial

lighting would hardly be begun did not deter him in the slightest. To one of Edison's

enthusiastic self-confidence the long vista of difficulties ahead--we say it in all sincerity--

must have been alluring.

After having devoted several months to experimental trials of carbon, at the end of 1878,

as already detailed, he turned his attention to the platinum group of metals and began a

series of experiments in which he used chiefly platinum wire and iridium wire, and alloys

of refractory metals in the form of wire burners for incandescent lamps. These metals

have very high fusing-points, and were found to last longer than the carbon strips

previously used when heated up to incandescence by the electric current, although under

such conditions as were then possible they were melted by excess of current after they

had been lighted a comparatively short time, either in the open air or in such a vacuum as

could be obtained by means of the ordinary air-pump.

Nevertheless, Edison continued along this line of experiment with unremitting vigor,

making improvement after improvement, until about April, 1879, he devised a means

whereby platinum wire of a given length, which would melt in the open air when giving a

light equal to four candles, would emit a light of twenty-five candle-power without

fusion. This was accomplished by introducing the platinum wire into an all-glass globe,

completely sealed and highly exhausted of air, and passing a current through the platinum

wire while the vacuum was being made. In this, which was a new and radical invention,

we see the first step toward the modern incandescent lamp. The knowledge thus obtained

that current passing through the platinum during exhaustion would drive out occluded

gases (i.e., gases mechanically held in or upon the metal), and increase the infusibility of

the platinum, led him to aim at securing greater perfection in the vacuum, on the theory

that the higher the vacuum obtained, the higher would be the infusibility of the platinum

burner. And this fact also was of the greatest importance in making successful the final

use of carbon, because without the subjection of the carbon to the heating effect of

current during the formation of the vacuum, the presence of occluded gases would have

been a fatal obstacle.

Continuing these experiments with most fervent zeal, taking no account of the passage of

time, with an utter disregard for meals, and but scanty hours of sleep snatched reluctantly

at odd periods of the day or night, Edison kept his laboratory going without cessation. A

great variety of lamps was made of the platinum-iridium type, mostly with thermal

devices to regulate the temperature of the burner and prevent its being melted by an

excess of current. The study of apparatus for obtaining more perfect vacua was

unceasingly carried on, for Edison realized that in this there lay a potent factor of ultimate

success. About August he had obtained a pump that would produce a vacuum up to about

the one-hundred- thousandth part of an atmosphere, and some time during the next

month, or beginning of October, had obtained one that would produce a vacuum up to the

one-millionth part of an atmosphere. It must be remembered that the conditions necessary

for MAINTAINING this high vacuum were only made possible by his invention of the

one-piece all-glass globe, in which all the joints were hermetically sealed during its

manufacture into a lamp, whereby a high vacuum could be retained continuously for any

length of time.