ceased his efforts until a practical incandescent lamp had been produced. His idea was to perfect a lamp that would do everything that gas could do, and more; a lamp that would give a clear, steady light, without odor, or excessive heat such as was given by the arc lights-in short, a household lamp.

Early in his experiments he abandoned the voltaic arc, deciding that a successful lamp must be one in which incandescence is produced by a strong current in a conductor, the heat caused by the resistance to the current producing the glow and light. But when search was made for a suitable substance possessing the necessary properties to be the incandescent material, the inventor was confronted by a vast array of difficulties. It was of course essential that the substance must remain incandescent without burning, and at the same time offer a resistance to the passage of the current precisely such as would bring about the heating that produced incandescence. It should be infusible even under this high degree of heat, or otherwise it would soon disappear; and it must not be readily oxidizable, or it would be destroyed as by ordinary combustion. It should also be of material reducible to a filament as fine as hair, but capable of preserving a rigid form. These, among others, were the qualities to be considered in selecting this apparently simple filament for the incandescent lamp. It was not a task for the tyro, therefore, that Edison undertook when he began his experiments for producing an "ideal lamp."

The substance in nature that seemed to possess most of the necessary qualities just enumerated was the metal

platinum, and Edison began at once experimenting with this. He made a small spiral of very fine platinum wire, which he enclosed in a glass globe about the size of an ordinary baseball. The two ends of the wires connected with outside conducting wires, which were sealed into the base of the bulb. The air in the bulb had to be exhausted and a vacuum maintained to diminish the loss of heat and of electricity and to prevent the oxidation of the platinum. But when the current was passed through the spiral wire in this vacuum a peculiar change took place in the platinum itself. The gases retained in the pores of the metal at once escaped, and the wire took on such peculiar physical properties that it was supposed for a time by some physicists that a new metal had been produced. The metal acquired a very high degree of elasticity and became susceptible of a high polish like silver, at the same time becoming almost as hard as steel. It also acquired a greater calorific capacity so that it could be made much more luminous without fusing. To diminish the loss of heat the wire was coated with some metallic oxide, and the slope of the spiral also aided in this as each turn of the spiral radiated heat upon its neighbor, thus utilizing a certain amount that would otherwise have been lost. But despite all this, Edison found, after tedious experimenting, that platinum did not fulfil the requirements of a practical filament for his lamp; it either melted or disintegrated in a short time and became useless; and the other experimenters had met with the same obstacles to its use, and were forced to the same conclusion.

Some other substance must be found. The use of carbon for arc lights and Edison's own experiments with carbon in his work on the telephone naturally suggested this substance as a possibility. It is said that this idea was brought forcibly to the inventor's attention by noticing the delicate spiral of vegetable carbon left in his hand after using a twisted bit of paper, one day, for lighting a cigar. This spiral of carbon was, of course, too fragile to be of use in its ordinary form. But it occurred to Edison that if a means of consolidating it could be found, there was reason to hope that it would answer the purpose. Experiments were begun at once, therefore, not only with processes of consolidation but also with various kinds of paper, and neither effort nor expense was spared to test every known variety of paper. Moreover, many new varieties of paper were manufactured at great expense from substances having peculiar fibres. One of these, made from a delicate cotton grown on some little islands off South Carolina, gave a carbon free from ash, and seemed to promise good results; but later it was found that the current of electricity did not circulate through this substance with sufficient regularity to get protracted and uniform effects. Nevertheless, since many things pointed to this fibre carbon as the ideal substance, Edison set about determining the cause of the irregularity in the circulation of the current in the filament, and a number of other experimenters soon became interested in the problem.

It was soon determined that the arrangement of the fibres themselves were directly responsible for the difficulty. In ordinary paper the fibres are pressed to