Page 270 - Electrical Properties of Materials

252 Dielectric materials

of such a filter has already been given in Section 10.5. Two further realizations
in integrated optics form will be discussed in Section 13.7).
8
Up to now about 6×10 km of fibre been laid. The rate of increase halved in
2002 but that was believed to be due to the recession rather than to the coming
saturation of the market, and the economic slump continues.
In conclusion, it is worth mentioning that the reduction of attenuation by
four orders of magnitude in less than two decades is about the same feat as
was achieved by strenuous efforts between the first attempts of the Phoeni-
cians (around 2000 BC) and the dawn of the fibre age. If you ever encounter a
problem that appears to be too daunting, remember the story of optical fibres.
It is an excellent illustration of the American maxim (born in the optimism of
the post-war years) that the impossible takes a little longer.

10.16 The Xerox process
This great development of the past three decades enables the production of high
quality reproductions of documents quickly and easily. This has in turn made
decision making more democratic, bureaucrats more powerful, and caused vast
forests of trees to disappear to provide the extra paper consumed. Scientifically,
the principles are simple. The heart of the Xerox machine is a plate made of a
thin layer of amorphous semiconductor on a metal plate. The semiconductor is
a compound of As, Se, and Te. It is almost an insulator, so that it behaves like
a dielectric, but it is also photo-conductive; that is, it becomes more conduct-
ing in the light (remember Section 8.6). The dielectric plate is highly charged
electrostatically by brushing it with wire electrodes charged to about 30 kV.
The document to be copied is imaged onto the plate. The regions that are white
cause the semiconductor to become conducting, and the surface charge leaks
away to the earthed metal backing plate. However, where the dark print is im-
aged, charge persists. The whole plate is dusted with a fine powder consisting
of grains of carbon, silica, and a thermosetting polymer. Surplus powder is
shaken off, and it adheres only to the highly charged dark regions. A sheet of
paper is then pressed onto the plate by rollers. It picks up the dust particles
and is then treated by passing under an infrared lamp. This fuses polymeric
particles, which subsequently set, encasing the black C and SiO 2 dust to form
a permanent image of the printed document. To clear the plate, it is illuminated
all over so that it all discharges, the ink is shaken off, and it is ready to copy
something else.
A very simple process scientifically, it works so well because of the careful
and very clever technological design of the machine.

10.17 Liquid crystals

I suppose we have heard so much about liquid crystal displays (LCD) in the
last couple of decades that we tend to ignore the implied contradiction. Is it
a liquid, or is it a crystal? Well, it can be both, and the fact has been known
for nearly a hundred years. This particular set of viscous liquids happens to
have anisotropic properties due to ordering of long rod-like molecules. If you
would like to visualize the short-range order of long rods have a look at the
photograph shown in Fig. 10.26 taken in Canada by Dr Raynes, an expert on