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Selenium photos of resistance
      An internal photoeffect was open in 1873, and in two years the first selenium photo was made resistance.
      Sometimes the semiconductor photos of resistance not fully successfully name photocells. In semiconductor resistance the eaten up a radiation causes diminishing of resistance of matter and creates neither a photo-electric current nor E.m.f , as in a photocell with a locking layer.
      Between a photocell with an external photoeffect and photo resistance also is fundamental distinction. In a vacuum or gasfilled photocell a current can go only one-way. Resistance is deprived a photo this feature. It identically installs electrical an equipment both in one and in other direction. At an internal photoeffect, as be already shown in a preceding chapter, energy of eaten up light is expended not on breaking of electrons out of matter, and on liberation some their numbers into mass of matter, on translation of them in the areas of conductivity. Due to appearance of photo of electrons electric resistance of semiconductor which photos of resistance is made out of diminishes notedly. Here radiant energy does not prove in an obvious kind, and is in latency, as canned in material of photo of resistance. Thus, photos of resistance is a semiconductor device, changing the electric resistance under the action of light.
      From previous consideration of physical essence of internal photoeffect the fundamental device of photo of resistance becomes clear. Resistance is a plate (in most cases skim of semiconductor) a photo, on the regional areas of which metallic electrodes, providing a reliable electric contact, are laid on. Such photo resistance joins in a chain consistently with the source of feed.
      When resistance is black-out a photo, in all chain, and, consequently, and in a photo resistance, an electric current the size of which is determined by ohme resistance of photo of resistances and difference of potentials attached to him flows. Such current is named darkly. At illumination the photo of resistance a current increases and growth it the more than anymore light stream. Difference between light and darkly currents and gives the value of photoelectric to us.
      The photos of resistance depending on setting are made from different semiconductor materials. Sensitiveness   of photo of resistances   considerably   higher of sensitiveness of vacuum photocells with an external photoeffect. It means that at identical luminosity a photo resistance provides in several times greater growth of current in a chain as compared to a photocell with an external photoeffect. The substantial lack of majority of photo of resistances is a considerable inertance, while a vacuum photocell with an external photoeffect is practically a fast-response device.
      The second substantial lack of photo of resistances is nonlinear dependence of growth of photoelectric on growth of intensity of light stream. On figs. 1 dependence over of photoelectric is brought in microamperes (мка) from a light stream in lumens (lm).
      These two defects are very substantial although, they do not close a photo to resistances of access in many areas of technique. In addition, new semiconductor materials which would be largely deprived these defects are presently found, the new types of photos of resistance are constructed with the improved parameters. In same for some time past physicists succeeded to settle many difficulties and get a photo resistances, in a great deal free of defects, inherent to their predecessors.
      Depending on a matter, out of which photos are made resistances, last possess different spectral descriptions and different integral sensitiveness.    Under   the integral    sensitiveness of photocell or photo of resistance understand the size of photoelectric, which flows in a chain at falling on the photocell of unit of stream of radiant energy, consisting of waves of different length and corresponding on the spectral composition to the radiation of lamp with a tungsten filament which is incandescent to the temperature 2 840° К. This unit of stream of visible light is named  lumen. Thus, the integral sensitiveness of photocell in visible part of spectrum is measured in microamperes on a lumen (mkA/lm). A spectral sensitiveness characterizes the size of photoelectric, arising up from the action of unit of stream of radiant energy of certain wave-length. Thus, if the integral sensitiveness of photocell can be determined by one number - size of photoelectric, attributed to unit of energy of falling radiation, then the spectral sensitiveness of every photocell is usually represented as a chart. On the vertical axis of chart the values of sensitiveness or photoelectric are put aside, and on a horizontal axis are values of length of waves in some units, for example in angstreme.
      The first at times photos of resistance was made from selenium, playing the largest role in history of science and technique. Technology of making of selenium photos of resistances was developed  and perfected  for years.

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See also:
Temperature sensors, thermocouple             Sensors of mechanical stress
Force, pressure, flow                                    Phototransducers
Ionization transducers                                    Hall sensors
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