The Invention Of The Incandescent Lamp - 7

The Invention Of The Incandescent Lamp - 7

If we consider a simple circuit in which a current is flowing, and include in the circuit a

carbon horseshoe-like conductor which it is desired to bring to incandescence by the heat

generated by the current passing through it, it is first evident that the resistance offered to

the current by the wires themselves must be less than that offered by the burner, because,

otherwise current would be wasted as heat in the conducting wires. At the very

foundation of the electric- lighting art is the essentially commercial consideration that one

cannot spend very much for conductors, and Edison determined that, in order to use wires

of a practicable size, the voltage of the current (i.e., its pressure or the characteristic that

overcomes resistance to its flow) should be one hundred and ten volts, which since its

adoption has been the standard. To use a lower voltage or pressure, while making the

solution of the lighting problem a simple one as we shall see, would make it necessary to

increase the size of the conducting wires to a prohibitive extent. To increase the voltage

or pressure materially, while permitting some saving in the cost of conductors, would

enormously increase the difficulties of making a sufficiently high resistance conductor to

secure light by incandescence. This apparently remote consideration --weight of copper

used--was really the commercial key to the problem, just as the incandescent burner was

the scientific key to that problem. Before Edison's invention incandescent lamps had been

suggested as a possibility, but they were provided with carbon rods or strips of relatively

low resistance, and to bring these to incandescence required a current of low pressure,

because a current of high voltage would pass through them so readily as not to generate

heat; and to carry a current of low pressure through wires without loss would require

wires of enormous size.[8] Having a current of relatively high pressure to contend with, it

was necessary to provide a carbon burner which, as compared with what had previously

been suggested, should have a very great resistance. Carbon as a material, determined

after patient search, apparently offered the greatest hope, but even with this substance the

necessary high resistance could be obtained only by making the burner of extremely

small cross-section, thereby also reducing its radiating surface. Therefore, the crucial

point was the production of a hair-like carbon filament, with a relatively great resistance

and small radiating surface, capable of withstanding mechanical shock, and susceptible of

being maintained at a temperature of over two thousand degrees for a thousand hours or

more before breaking. And this filamentary conductor required to be supported in a

vacuum chamber so perfectly formed and constructed that during all those hours, and

subjected as it is to varying temperatures, not a particle of air should enter to disintegrate

the filament. And not only so, but the lamp after its design must not be a mere laboratory

possibility, but a practical commercial article capable of being manufactured at low cost

and in large quantities. A statement of what had to be done in those days of actual as well

as scientific electrical darkness is quite sufficient to explain Tyndall's attitude of mind in

preferring that the problem should be in Edison's hands rather than in his own. To say

that the solution of the problem lay merely in reducing the size of the carbon burner to a

mere hair, is to state a half-truth only; but who, we ask, would have had the temerity even

to suggest that such an attenuated body could be maintained at a white heat, without

disintegration, for a thousand hours? The solution consisted not only in that, but in the

enormous mass of patiently worked-out details--the manufacture of the filaments, their

uniform carbonization, making the globes, producing a perfect vacuum, and countless

other factors, the omission of any one of which would probably have resulted eventually

in failure.

[8] As a practical illustration of these facts it was calculated by Professor Barker, of the

University of Pennsylvania (after Edison had invented the incandescent lamp), that if it

should cost $100,000 for copper conductors to supply current to Edison lamps in a given

area, it would cost about $200,000,000 for copper conductors for lighting the same area

by lamps of the earlier experimenters --such, for instance, as the lamp invented by Konn

in 1875. This enormous difference would be accounted for by the fact that Edison's lamp

was one having a high resistance and relatively small radiating surface, while Konn's

lamp was one having a very low resistance and large radiating surface.