X-RAYS, AND HOW THE BONES OF THE HUMAN BODY ARE PHOTOGRAPHED.
[Illustration: Fig. 216.]
[Illustration: Fig. 217.]
=154. Disruptive Discharges.= We have seen, in the study of induction
coils, that a spark can jump several inches between the terminals
of the secondary coil. The attraction between the two oppositely
charged terminals gets so great that it overcomes the resistance of
the air-space between them, a brilliant spark passes, and they are
discharged. This sudden discharge is said to be _disruptive_, and it
is accompanied by a flash of light and a loud report. The _path_ of
the discharge may be nearly straight, or crooked, depending upon the
nature of the material in the gap between the terminals.
[Illustration: Fig. 218.]
[Illustration: Fig. 219.]
=155. Effect of Air Pressure on Spark.= The disruptive spark takes
place in air at ordinary pressures. The nature of the spark is greatly
changed when the pressure of the air decreases. Fig. 216 shows an
air-tight glass tube so arranged that the air can be slowly removed
with an air-pump. The upper rod shown can be raised or lowered to
increase the distance between it and the lower rod, these acting as the
terminals of an induction coil. Before exhausting any air, the spark
will jump a small distance between the rods and act as in open air. As
soon as a small amount of air is removed, a change takes place. The
spark is not so intense and has no definite path, there being a general
glow throughout the tube. As the air pressure becomes still less, the
glow becomes brighter, until the entire tube is full of purple light
that is able to pass the entire length of it; that is, the discharge
takes place better in rarefied air than it does in ordinary air.
=156. Vacuum-Tubes.= As electricity passes through rarefied gases much
easier than through ordinary air, regular tubes, called _vacuum-tubes_,
are made for such study. Fig. 217 shows a plain tube of this kind,
platinum terminals being fused in the glass for connections. These
tubes are often made in complicated forms, Fig. 218, with colored
glass, and are called _Geissler tubes_. They are often made in such a
way that the electrodes are in the shape of discs, etc., and are called
_Crookes tubes_, Fig. 219. A slight amount of gas is left in the tubes.
[Illustration: Fig. 220.]
[Illustration: Fig. 220-A.]
=157. Cathode Rays.= The _cathode_ is the electrode of a vacuum-tube
by which the current leaves the tube, and it has been known for some
time that some kind of influence passes in straight lines from this
point. Shadows, Fig. 219, are cast by such rays, a screen being placed
in their path.
=158. X-Rays.= Professor Roentgen of Würzburg discovered that when the
cathode rays are allowed to fall upon a solid body, the solid body
gives out still other rays which differ somewhat from the original
cathode rays. They can penetrate, more or less, through many bodies
that are usually considered opaque. The hand, for example, may be used
as a negative for producing a photograph of the bones, as the rays do
not pass equally well through flesh and bone.
[Illustration: Fig. 221.]
Fig. 220 shows a Crookes tube fitted with a metal plate, so that
the cathode rays coming from C will strike it. The X-rays are given
out from P. These rays are invisible and are even given out where
the cathode rays strike the glass. Some chemical compounds are made
luminous by these rays; so screens are made and coated with them in
order that the shadows produced by the X-rays can be seen by the
eye. Professor Roentgen named these the X-rays. Fig. 220-A shows a
_fluoroscope_ that contains a screen covered with proper chemicals.
[Illustration: Fig. 222.]
[Illustration: Fig. 223.]
=159. X-Ray Photographs.= Bone does not allow the X-rays to pass
through it as readily as flesh, so if the hand be placed over a
sensitized photographic plate, Fig. 221, and proper connections be
made with the induction coil, etc., the hand acts as a photographic
negative. Upon developing the plate, as in ordinary photography,
a picture or shadow of the bones will be seen. Fig. 222 shows the
arrangement of battery, induction coil, focus tube, etc., for examining
the bones of the human body.
Fig. 223 shows the bones of a fish. Such photographs have been very
valuable in discovering the location of bullets, needles, etc., that
have become imbedded in the flesh, as well as in locating breaks in the
bones.