Man's physical universe

xanabras

!

550 MAGNETISM AND FXECTRICITY

than they are in personal gain.

In fact, a scientist can usually do his

best work only when his mind is completely free from financial worries.

In 1913 W. D. Coolidge improved the X-ray tube by using a coil of

wire heated to incandescence by an electric current to produce a stream

Fig. 273. Dental X-ray unit in

operation. (Courtesy of the General

Electric X-ray Corporation.)

be used in diagnosis.

of electrons, which were given a

high velocity by the application

of a potential of 100,000 volts to

1,500,000 volts between the filament

and the cold target. The exhaustion

of the air was carried to a

higher point than in previous tubes,

so that harder (more penetrating)

rays were obtained. By regulating

the voltage applied to the tube, the

'

' hardness " of the rays could be governed.

Theshorter the wavelength,

the more penetrating ("harder")

the rays are. Very high voltages

produce X rays of very high penetrating

power. Supervoltage X rays

require smaller exposures, and there

is less danger of burns than in

the

case of X-ray tubes of smaller voltage.

X rays are widely employed

in photographing the bones and

organs of the body. A considerable

detail of the soft tissues of the body

is also revealed, so that X rays can

Tuberculosis of the lungs, abscessed or abnormal

teeth, fractures, calculi (stones) in the kidneys, and many other abnormal

conditions can be detected by X rays.

The whole length of the

digestive canal can be studied by giving the patient a harmless dose

of bismuth carbonate or barium sulfate, which are opaque to X rays

and thus show the digestive organs in outline. X rays have to be

used with caution because they may cause severe burns that refuse

to heal. There were many martyrs to X rays before their harmful

properties were recognized and effective means of protection from

X rays were devised.

Besides their use in medicine. X-ray machines have been used in

detecting flaws in metals and welds; for example, every weld in the

huge pipes used in the Boulder Dam project was X-rayed by a 300,000-

volt X-ray machine, and twenty-nine miles of film were used in taking

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