Between the years 1841 and 1848 several inventors attempted to make light-sources by heating metals.  These crude lamps were operated by means of Grove and Bunsen electric cells, but no practicable incandescent filament lamps were brought out until the development of the electric dynamo supplied an adequate source of electric current.  As electrical science progressed through the continued efforts of scientific men, it finally became evident that an adequate supply of electric current could be obtained by mechanical means; that is, by rotating conductors in such a manner that current would be generated within them as they cut through a magnetic field.  Even the pioneer inventors of electric lamps made great contributions to electrical practice by developing the dynamo.  Brush developed a satisfactory dynamo coincidental with his invention of the arc-lamp, and in a similar manner, Edison made a great contribution to electrical practice in devising means of generating and distributing electricity for the purpose of serving his filament lamp.

[Illustration:  DIRECT CURRENT ARC

Most of the light being emitted by the positive (upper) electrode]

[Illustration:  FLAME ARC

Most of the light being emitted by the flame]

[Illustration:  ON THE TESTING-RACKS OF THE MANUFACTURER OF INCANDESCENT FILAMENT LAMPS

Thousands of lamps are burned out for the sake of making improvements.  The electrical energy used is equivalent to that consumed by a city of 30,000 inhabitants]

Edison in 1878 attacked the problem of producing light from a wire or filament heated electrically.  He used platinum wire in his first experiments, but its volatility and low melting-point (3200 deg.F.) limited the success of the lamps.  Carbon with its extremely high melting-point had long attracted attention and in 1879 Edison produced a carbon filament by carbonizing a strip of paper.  He sealed this in a vessel of glass from which the air was exhausted and the electric current was led to the filament through platinum wires sealed in the glass.  Platinum was used because its expansion and contraction is about the same as glass.  Incidentally, many improvements were made in incandescent lamps and thirty years passed before a material was found to replace the platinum leading-in wires.  The cost of platinum steadily increased and finally in the present century a substitute was made by the use of two metals whose combined expansion was the same as that of platinum or glass.  In 1879 and 1880 Edison had succeeded in overcoming the many difficulties sufficiently to give to the world a practicable incandescent filament lamp.  About this time Swan and Stearn in England had also produced a successful lamp.

In Edison’s early experiments with filaments he used platinum wire coated with carbon but without much success.  He also made thin rods of a mixture of finely divided metals such as platinum and iridium mixed with such oxides as magnesia, zirconia, and lime.  He even coiled platinum wire around a piece of one of these oxides, with the aim of obtaining light from the wire and from the heated oxide.  However, these experiments served little purpose besides indicating that the filament was best if it consisted solely of carbon and that it should be contained in an evacuated vessel.