Студопедия — Hopping electron sand the biggest disappointment of the television industry
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Hopping electron sand the biggest disappointment of the television industry






It is well known that when an electric field in a vacuum points along a glass surface, electrons can hop along the glass surface. The general effect is usually unwelcome. Among others, the hopping effect is responsible for sparks in vacuum systems that contain high voltage. When this effect was studied in detail, it turned out that reasonably low electric fields are sufficient to create sizeable electric hopping currents. The effect also works around bends and corners. Furthermore, electric switches that change the hopping direction can be constructed. In short, the hopping effect can be used to make extremely cheap flat television displays of high image quality. The idea is to put an array of electron sources –essentially sharp metal tips – at the start of glass channels and to transport the emitted electrons along the channels, making use of suitable switches, until they hit phosphorescent colour pixels.

These are the same pixels that were used in the then common – bulky and heavy – television tubes and that are used today in flat plasma displays. Since the hopping effect also works around bends and corners, and since it only needs glass and a bit of metal, the whole system can be made extremely thin, lightweight and cheap. Already in the early 1990s, the laboratory samples of the electron hopping displays were spectacularly good: the small displays were brighter, sharper and cheaper than liquid crystal displays, and the large ones brighter, sharper and cheaper than plasma displays.

Affordable flat television was on the horizon Then came the disappointment. The lifetime of the displays was only of the order of one hundred hours. Despite the most intense material research possible, achieving a higher lifetime turned out to be impossible. All tricks that were tried did not help. Despite all their fantastic properties, despite huge investments in the technology, despite the best material researchers working on the issue, electron hopping displays could not be brought to market. Not a single display was ever sold.

 

How do nerves work?

Nerves are wonders. Without nerves, we would not experience pleasure, we would not experience pain, we would not see, we would not hear. Without nerves, we would not live. But how do nerves transport signals?

In 1789 Luigi Galvani discovered that nerves transport electric signals, by doing experiments with frog legs. Are nerves wires? One and a half centuries after Galvani it became clear that nerves do not conduct electricity using electrons, as metal wires do, but by using ions. Nerve signals propagate using the motion of sodium Na + and potassium K + ions through the cell membrane of the nerve. The resulting signal speed is between 0.5 m /s and 120 m/s, depending on the type of nerve. (Nerve axons coated with myelin, a protein that acts as an electric insulator, are faster than uncoated axons.) The signal speed is sufficient for the survival of most species – it helps the body to run away in case of danger.

Nerves differ from wires in another aspect: they cannot transmit constant voltage signals, but only signal pulses. The first, approximate model for this behavior was presented in 1952 by Hodgkin and Huxley. Using observations about the behaviour of potassium and sodium ions, they deduced an elaborate evolution equation that describes the voltage V in nerves, and thus the way the signals propagate. The equation reproduces the characteristic voltage spikes measured in nerves.

The precise mechanism with which ions cross the membranes, using so-called channel proteins, was elucidated only twenty years later. Despite this huge body of work, and even though Hodgkin and Huxley received the Nobel Prize for Medicine for their work, their model cannot be correct. The model does not explain the reversibility of the propagation process, the observed thickness change of the nerve during propagation or the excitation of nerves by simple deformation or temperature changes; most of all, the model does not explain the working of anesthetics. The working of nerves remained unknown.

Only around the year 2000 did Thomas Heimburg and his team discover the way signals propagate in nerves. They showed that a nerve pulse is an electromechanical solitonic wave of the cylindrical membrane. In the cylindrical membrane, the protein structure changes from liquid to solid and back to liquid. A short, slightly thicker ring of solid proteins propagates along the cylinder: that is the nerve pulse. (The term ‘solid’ has a precise technical meaning in two-dimensional systems and describes a specific ordered state of the molecules.) This model explains all the properties of nerve pulses that were unexplained before. In particular, it explains that anesthetics work because they dissolve in the membrane and thus block the formation and the propagation of the rings. All quantitative predictions of the model match observations.

In short, nerve signals are electromechanical pulses; they are a mixture of current and sound waves. The electromechanical model of nerves explains how signals propagate and how pain is felt. The model also explains why no pain is felt during anesthesia. On the other hand, the electromechanical model does not (yet) explain why we loose consciousness during anesthesia. This is an additional process that takes place in the brain. It is known that loss of consciousness is related to the change of brain waves, but the details are still a topic of research. Nerves and brains still have wonderful properties to be explored.

 

 

Check yourself:

1. Give English equivalents for the following:

постоянный ток, единица величины тока, направление, проводить, проводник, проводимость, сталкиваться, столк­новение, количество электричества, сечение проводника, со­противление, температурный коэффициент сопротивления, электрическое иоле, магнитное поле, амперметр, повышать, понижать.

 







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