- × or x or X
- the usual symbol for power as a unit of
magnification. More generally, × or x is used with its
mathematical meaning, "times," to indicate
that a measurement is a multiple of some standard or reference measurement.
For example, an image marked 200× is shown at 200 times actual size (that
is, distances are 200 times actual size), and a 10x CD-ROM drive is one
capable of transferring data 10 times faster than the "normal" or standard
- X unit (Xu)
- a unit of distance formerly used for measuring the wavelength
of x-rays and gamma rays. The X unit is approximately 1.0021 x
10-13 meter, 0.001 0021 angstrom,
or 100.21 femtometers. The wavelength of these powerful forms of
radiation is now measured in picometers (pm) or femtometers (fm).
The unit was defined by the Swedish physicist K. M. G. Siegbahn in
1925, at a time when the wavelengths could not be measured
directly. The definition was made in terms of the spacing between
planes of the calcite crystals used in the measuring apparatus.
Siegbahn, who received the Nobel Prize in Physics in 1924 for his
work in X-ray spectroscopy, aimed to define a unit equal to
approximately 10-13 meter, and he succeeded admirably.
In his honor the unit was also called the Siegbahn
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March 17, 2000