Edisonov učinak

Johnson piše:

"...Godina je bila 1880. Thomas A. Edison bio je zaokupljen radom u svom laboratoriju u Menlo Parku razvijajući žarulju s ugljenom niti. Žarulja još nije bila u prodaji, ali se činila vrlo obečavajućom. U to doba žarna nit je načinjena od potkovasto izrezanog komada kartona koji je zatim pougljenjen i učvršćen na stakleni stupić. Sve je zatim zataljeno u staklenu "krušku" iz koje je pomoću živine sisaljke isisan zrak. "Sa žaruljama je postojala jedna poteškoća, u tome što se ugljik iz žarne niti taložio na unutrašnju stjenku staklene "kruške" i to zacrnjenje smanjivalo je učinak i vijek trajanja žarulje. Kako je dolazilo do tog zacrnjenja, i kako se to moglo spriječiti? O tome su mnogo spekulirali i ispitivali. A onda je netko primijetio da pozitivan kraj žarne niti baca sjenu na staklo, kao da ugljik dolazi pravocrtno s negativnog kraja niti, a pozitivan mu je kraj presjekao put. No, godinu dana ranije Crookes je pokazao da su katodne zrake negativno nabijene čestice koje pri izboju u plinu idu od negativne prema pozitivnoj elektrodi. To je navelo Edisona i njegove ljude na pomisao da se iz žarne niti u žarulji izbacuju negativno nabijeni atomi ugljika and only those from the negative end could get to the glass. A shadow would be left by the positive end. (There was no alternating current then, Edison was always a dc man.) The projection of carbon was called "electrical carrying," on the assumption that the carbon carried the current or, perhaps, vice versa. They ascribed it to the Crookes effect, still with some misgivings. It was to lead them on a long, wrong trail, yet with interesting experimental results.

"Now, if the carbon particles are charged it should be possible to draw them to a separate electrode, away from the glass. Furthermore, it should be possible to measure the electric current to this electrode. On February 13, 1880, Edison entered in his notebook the first of a long series of experiments on prevention of carrying. The drawing shows a lamp with the "horseshoe" filament, and an extra electrode inserted, a piece of platinum foil. The test was made by Charles Batchelor, one of Edison's engineers....

"A later entry that year, November 28, shows two added electrodes, differently disposed, for the "prevention of carrying." In July, 1882, the problem was still worked at. Here there was a considerable break in these tests, and this is understandable. On September 4, 1882 Edison opened his Pearl Street station in New York, the world's first generating station for supplying power to the new electric lamps. He had invented, designed, and built the generators, distribution lines, meters, switches, fuses, and practically all the things that go with electric distribution. He had been busy. Then again on March 9, 1883, there is an entry by Edison suggesting that magnetic fields might be effective. The lamp was pumped by Martin Force (whose brother, Joe, also worked for Edison and who, years later, worked with me as a glass blower), and tested by John Ott. The results were evidently still discouraging so far as carrying was concerned, and other directives followed, suggesting various electrode materials, the effect of strong light, magnetic fields, etc. One note directed Acheson (later of graphite and carborundum fame) to 'Try Everything.'

"An entry by John Ott on October 8, 1883, is interesting. Edison had asked him to set up a lamp with the extra electrode connected through a galvanometer to one side of the line. The purpose was to see if the current in this branch circuit could be used to show the voltage of the line. While this seems to be the first entry showing a galvanometer in the circuit, it is pretty certain that the current to the extra electrode had been measured before and found related to the temperature of the filament. This is the first application of thermionic current to a useful purpose. It is the first electronic circuit, with the first thermionic tube.

"Edison saw a potential usefulness in such a scheme and, as was his custom, immediately applied for a patent on it, that was granted less than a year later....

"Edison, having thus informed the world of his discovery in a patent, proceeded also to do it in other ways. In the fall of 1884 there was an International Electrical Exposition in Philadelphia, held at the Franklin Institute and attended by many prominent people from different lands. Edison had a number of exhibits there, among them a model of his voltage regulator. The exhibits evidently had cards with them to explain their operation....

"In connection with the Exposition there was held a meeting of the American Institute of Electrical Engineers, October 7-8, 1884. Edison had a paper on his discovery prepared and presented at this meeting by Professor E. J. Houston....This paper was published beginning on page 1 of Volume 1 of the Transactions of the American Institute of Electrical Engineers....In the text Professor Houston describes the experiment on current flow to a platinum plate in the lamp. The diagram shows the line or battery, the lamp with the filament and platinum plate, and the galvanometer connected to one or the other side of the line. He describes the experiments in detail, and is puzzled by them. Particularly he wonders about the origin of the current that flows in the plate circuit. He says the vacuum is nearly complete, so he 'cannot conceive of current flowing across the vacuous space, as this is not in accordance with our preconceived ideas connected with high vacua....I have no theory to propound as to the origin of these phenomena.' Again: 'It may be electricity flowing through empty space, which I don't think probable.' He said it could be a Crookes effect, charged molecules flowing from plate to loop; but the direction of the current is wrong for this. 'For my own part I am somewhat inclined to believe that we may possibly have here a new source of electrical excitement. That in some way the molecular bombardment against the platinum plate may produce an electrical current.'

"Among those present at the meeting was the eminent chief engineer of the British Post Office's telegraph service, Sir William H. Preece. In the discussion Preece said he had seen the exhibit and was puzzled by it. He thought there might be here a new source of electricity, but in his own mind he considered 'the cause of this remarkable phenomena (sic) to be due to the Crookes effect....

"'Every other electrician present at the Exhibition, I think, has watched this experiment with great interest. I feel puzzled in reference to it, and I feel that it is one of those things that wants to be very carefully and cautiously examined. [How right he was!] I intend to exercise my persuasive eloquence upon Mr. Edison when I see him next week to induce him to give me one of those lamps, and when I go back to England I shall certainly make an illustration before our society there, and then make careful inquiry into it.'

"He did see Edison, who gave him several of his lamps with the added electrode. He published a paper the following spring, in the March, 1885 issue of the Proceedings of the Royal Society, entitled 'On a peculiar behavior of glow lamps when raised to high incandescence.' In this paper Preece was the first to use the term 'Edison effect.' ...Still misunderstanding what Crookes had observed, he assumed that the blackening was a consequence of the Crookes effect.

"Next on the British scene was J. A. Fleming, then consultant to the Edison Electric Light Company, London. In late 1885 he published a paper in the Philosophical Magazine, 'On molecular shadows in incandescent lamps.' He had undoubtedly received Preece's message, but he claimed he had independently noticed the blackening and shadows in 1882, with about the same observations as Edison. He still had in mind these mysterious experiments, when in 1896 he published a 50-page paper in the Philosophical Magazine entitled, 'A further examination of the Edison effect in glow lamps.' In this paper he proved to his own satisfaction that a good vacuum is not a conductor, and that carriers are needed for the passage of current. The carriers he supposed to be the carbon particles, again the wrong interpretation of the Crookes effect. The trouble was that he was a little too early. The discovery of the electron was only two years away.

"Crossing the Atlantic again, in 1897 an Edison engineer, J. W. Howell, read before the A.I.E.E. a paper on the 'Conductivity of incandescent carbon filaments and the space surrounding them.' This was work done at the Edison laboratory in collaboration with another Edison engineer, A. E. Kennelly. After discussing the filaments themselves he went on to the 'Edison effect.' They had used alternating current in the filament, and found then that both ends of the filament cast a white shadow. The current was still considered as carried by negatively charged molecules of carbon. Then they found that the current to the collecting electrode was the same when connected to either side of the filament. They had discovered rectification, and this fact was discussed by Kennelly and others.

"This was about the state of the Edison effect just before the discovery of the electron. No more papers were published on the effect as it appeared in carbon filament lamps....

"...First: What was the outcome of the Edison experiments on blackening? I have not seen an answer given directly but I think I know what it is. Putting added electrodes in the lamps did not solve the blackening problem. The problem probably lay in the gradually improved vacuum (getters were introduced around 1890), and in running the lamps at a low enough temperature so blackening was not serious....

"The second question is: What was it that happened in the Edison and other experiments on lamps, that so puzzled the experimenters? I have seen no attempt at an explanation, so again we are left to work out one for ourselves, and it will be quite different from what the early experimenters had the background even to surmise. We know that evaporation from hot bodies such as carbon takes place by emission of neutral carbon atoms, not by charged particles. This means that the current was carried principally by electrons and not by negative ions. The vacuum they had was none too good, even if they called it a millionth of an atmosphere. The density of gas molecules was undoubtedly vastly greater than that of the evaporating carbon atoms. With a space current flowing, of some milliamperes at 100 volts or more, there would be considerable ionization of this gas, producing additional electrons, and ions with positive charge, not negative. The positive gas ions would bombard the negative end of the filament, and there give rise to emission of carbon atoms by the sputtering process. At this relatively low pressure both the evaporated and the sputtered atoms would go in straight lines from the filament to be deposited on the walls. But if sputtering exceeds evaporation at the negative end of the filament, then there would be a partial shadow cast by the positive end of the filament, just as if negative ions were emitted by the negative end. At the same time, in the flow of the electron current the negative end of the filament would be cooled by the emission of electrons, and the positive end would be heated by the electron bombardment. This would explain the difference in temperature sometimes observed between the two ends of the filament....

"This explanation seems now fairly obvious and simple. But between it and the early observations there lies a series of experimental and theoretical works that were the major life work of many eminent scientists.

"...The discovery of large currents flowing from a hot filament was not something that was recorded in a notebook and forgotten. Edison told people about it, he applied for a patent describing and making use of it, he caused it to be reported in the scientific literature, and it aroused a lively interest among eminent men of the day. The explanation of the phenomenon had to wait a couple of decades for the revolutionary ideas of Thompson, Drude, and Richardson. The truth then turned out to be that the carriers of negative charge were not carbon atoms in Edison's experiments or other gas molecules in the Crookes effect, but electrons in both. Edison's contribution was not to explain the phenomenon, but to create and keep alive a keen interest in the mysterious side effect that showed up in the blackening of lamps, the Edison effect."