Historic Inventions

by Rupert Sargent Holland

Then began the long months of testing platinum wire. He wanted to find some way of preventing this hardest of all metals from melting when the full force of the electric current was turned into it. He worked out several devices to keep the platinum from fusing, an automatic lever to regulate the electric current when the platinum was near the melting-point, and a diaphragm with the same object; but all of them had to be discarded. Although he was still searching for the right clue he seems to have had no doubt of his final success. He said at this time, “There is no difficulty about dividing up the current and using small quantities at different points. The trouble is in finding a candle that will give a pleasant light, not too intense, which can be turned off and on as easily as gas. Such a candle cannot be made from carbon points, which waste away, and must be regulated constantly while they do last. Some composition must be discovered which will be luminous when charged with electricity and that will not wear away. Platinum wire gives a good light when a certain quantity of electricity is passed through it. If the current is made too strong, however, the wire will melt. I want to get something better.”

It was generally known that Edison was working along this line. An English paper, commenting on the matter, said, “The weak point of the lamp is this, that in order to be luminous, platinum must be heated almost to the point of melting. With a slight increase in the current, the lamp melts in the twinkling of an eye, and in practice the regulator is found to short-circuit the current too late to prevent the damage. It is this difficulty which must be overcome. Can it be done?”

After long study Edison concluded that pure platinum was not suited to successful electric lighting. Then he incorporated with it another material of a non-conducting nature, with the result that when the electric current was turned on one material became incandescent and the other luminous. This gave a clear, but not a permanent, light. He tried many different combinations, and experimented month after month, but none of his trials produced the result he wanted, and at last he concluded that he was on the wrong track, and that neither platinum nor any other metal would give the right light.

There is something very dramatic about his real discovery. He was sitting in his laboratory one evening, when his right hand happened to touch a small pile of lampblack and tar that his assistants had been using in working on a telephone transmitter. He picked up a little of it, and began to roll it between his finger and thumb. He was thinking of other things, and he rolled the mixture absent-mindedly for some time, until he had formed a thin thread that looked something like a piece of wire. Glancing at it, he fell to wondering how it would serve as a filament for his light. It was carbon, and might be able to stand a stronger current than platinum. He rolled some more of the mixture, and decided to try it.

His experiments had already resulted in the production of an almost absolute vacuum, only one-millionth part of an atmosphere being left in the tube. Such a vacuum had never been obtained before. With his assistant, Charles Bachelor, he put a thread of the lampblack and tar in a bulb, exhausted the air, and turned on the current. There was an intense glow of light; but it did not last, the carbon soon burned out. Therefore he started to study the reason why the carbon had failed to withstand the electric current. His conclusion was that it was impossible to get the air out of the lampblack. Besides that the thread became so brittle that the slightest shock to the lamp broke it. But he felt certain now that a carbon filament, made of something other than tar and lampblack, was what he wanted.

He next sent a boy to buy a reel of cotton, and told his assistants he was going to see what a carbonized thread would do. They looked doubtful, but began the experiment. A short piece of the thread was bent in the form of a hairpin, laid in a nickel mould and securely clamped, and then put in a muffle furnace, where it was kept for five hours. Then it was taken out and allowed to cool. The mould was opened and the carbonized thread removed. It instantly broke. Another thread was put through the same process. As soon as it was taken from the mould it broke. Then a battle began that lasted for two days and two nights, the object of which was to get a carbonized thread that would not break. Edison wanted that thread because it contained no air, and might stand a greater current than the lampblack. Finally they took from the mould an unbroken thread, but as they tried to fasten it to the conducting wire it broke into pieces. Only on the night of the third day of their work, in all which time they had taken no rest, did they get a thread safely into the lamp, exhaust the air, and turn on the current. A clear, soft light resulted, and they knew that they had solved the problem of the incandescent light.

Edison and Bachelor watched that light for hours. They had turned on a small current at the start, to test the strength of the filament, but as it stood it, they turned on a greater and greater current until the thread was bearing a heat that would have instantly melted the platinum wire. The cotton thread glowed for forty-five hours, and then suddenly went out. The two watchers ended their long vigil, exhausted, but very happy. They knew that they had found the light that was to be the main illumination for the world.

But Edison realized that he had not yet found the ideal filament. The cotton thread had only lasted forty-five hours, and he wanted one that would burn for a hundred hours or longer. He wanted a more homogeneous material than thread, and he began to try carbonizing everything he could lay his hands on, straw, paper, cardboard, splinters of wood. He found that the cardboard stood the current better than the cotton thread, but even that did not burn long enough. Then he happened upon a bamboo fan, tore off the rim, and tried that. It made a filament that gave better results than any of the others.

Now he began his exhaustive study of bamboo. He learned that there were more than twelve hundred known varieties of bamboo. He wanted to find the most homogeneous variety. He sent out a number of men to hunt this bamboo, and it is said that the search cost nearly $100,000. Six thousand specimens of bamboo were carbonized, and he found three kinds of bamboo and one of cane that gave almost the result he wanted. All of these grew in a region near the Amazon, and were hard to get on account of malarial conditions. But at last he discovered the bamboo species that suited him, and he was ready to give his new light to the world.