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X-band

The X-band is the frequency band from 5200 to 10,900 mhz employed in radar.


Xanthine

Xanthine is a nitrogenous compound closely allied to uric acid, that occurs in

extract of meat and in tea. It forms a colourless powder slightly soluble in

water, and yields alloxan and urea on oxidation.


Xebec

A xebec is a small, sharp-built three-masted vessel with lateen sails. They

were used for coasting voyages in the Mediterranean and on the ocean coasts of

Spain and Portugal. It differs from the felucca in possessing both square and

lateen sails, the felucca having only lateen sails. The xebec was formerly much

used by Algerian pirates.


Xenon

Xenon is a rare, inert gaseous element. It's symbol is Xe.


Xerography

Xerography is an electrostatic dry-printing process for the reproduction of

images or documents, widely employed in commerce and industry in copying

machines, such as photo-copiers. The process was invented by the American

printer Chester F Carlson in 1937 and first commercially developed in 1950. It

makes use of the principle of photoconductivity, that is, that certain

substances resist passage of an electric current except when struck by light.

Silicon, germanium, and selenium are poor conductors of electricity, but when

light energy is absorbed by some of their electrons, the electrons are able to

pass from one atom to another, thus allowing a current to flow when a voltage

is applied. When the light is removed, their conductivity again becomes low.

Xerography employs a photoconductive insulating layer, such as selenium, on an

aluminum or other conductive metal support. The layer is charged

electrostatically, either with positive or negative ions ( see Ion ), the

polarity of the charge depending on the type of photoconductive insulating

layer selected. When the plate is exposed, in a camera or photographic machine,

those areas of the coating subjected to light lose a varying portion of the

charge, depending upon the intensity of the illumination. Thus, the variation

of the amount of charge retained on the coated metal plate is established as an

electrical or electrostatic pattern of the image. The image is rendered visible

by sprinkling over the exposed plate a special, charged powder, which carries

an opposite charge to the initial charge applied to the plate and insulating

layer. The powder adheres to those areas that have retained their charge. The

print is obtained by covering the plate with paper, then applying a charge over

the back of the paper of the same polarity as the initial charge applied to the

photoconductive insulating layer. In this way the opposite charged powders are

transferred to the paper surface. The powder image is then fused onto the

er by exposure to solvent vapors or heat to make the image permanent. The

entire xerographic process can be carried out, in high-speed mechanized

equipment, in less than 5 seconds, and it is comparatively inex pensive to make

these images because the photoconductive insulating layer can be recycled many

thousand times. The process has found its primary usefulness in copying office

documents and in low-volume duplication of data. The xerographic method also

permits the making, quickly and cheaply, of paper offset masterplates for

low-to-medium-volume runs on office offset-printing presses. The method has

also been applied to the production of X-ray images in a technique that is

known as xeroradiography and is used in mammography for the early detection of

breast cancer.


Xerox FormBase

Xerox FormBase is a computer forms management program. It combines forms

management with database techniques, which can be accessed from within the

Microsoft Windows environment. FormBase creates professional quality forms

while providing facilities to enter, sort, search, retrieve, and print data.

Unlike typical forms management packages which allow usars to create a single

form per file, FormBase lets you create multiple forms, or views, per database

file. Each form created is considered a different view of the database.

FormBase lets you create subforms and subtables within a form which facilitate

entering multiple entries into a field. For example, in a database containing

customer information you may want to create a data-entry form containing all

the invoices for each customer. Because the information varies for each

customer, this form may need to store dozens of entries. FormBase contains

features usually found in database packages. The product can perform a lookup

from one file to another. Information entered into one database can be

retrieved in another form. Mathematical computations can be built into a form

which are similar to formulae found in spreadsheet programs. Formulae can also

be created to validate information entered into forms to provide accuracy and

consistency during data entry. FormBase allows you to import graphic images

from other software programs into a database and to create forms with logos and

pictures. FormBase can print out columnar reports and supports laser,

dot-matrix, and colour printers.


Xylazine

see "Ketamine"


Xylophone

The xylophone is a percussion musical instrument.

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