THE RÖNTGEN RAYS
In the chapter on electricity in the atmosphere we saw that whereas
air at ordinary pressure is a bad conductor, its conducting power
increases rapidly as the pressure is lowered. Roughly speaking, if we
wish to obtain a spark across a gap of 1 inch in ordinary air, we must
have an electric pressure of about 50,000 volts. The discharge which
takes place under these conditions is very violent, and it is called
a “disruptive” discharge. If however the air pressure is gradually
lowered, the discharge loses its violent character, and the brilliant
spark is replaced by a soft, luminous glow.
The changes in the character of the discharge may be studied by means
of an apparatus known as the “electric egg.” This consists of an
egg-shaped bulb of glass, having its base connected with an air-pump.
Two brass rods project into the bulb, one at each end; the lower rod
being a fixture, while the upper one is arranged to slide in and out,
so that the distance between the balls can be varied. The outer ends of
the rods are connected to an induction coil or to a Wimshurst machine.
If the distance between the balls has to be, say, half an inch, to
produce a spark with the air at normal pressure, then on slightly
reducing the pressure by means of the air-pump it is found that a
spark will pass with the balls an inch or more apart. The brilliance
of an electric spark is due to the resistance of the air, and as the
pressure decreases the resistance becomes smaller, so that the light
produced is much less brilliant. If the exhaustion is carried still
further the discharge becomes redder in colour, and spreads out wider
and wider until it loses all resemblance to a spark, and becomes a
luminous glow of a purple or violet colour. At first this glow seems to
fill the whole bulb, but at still higher vacua it contracts into layers
of definite shape, these layers being alternately light and dark.
Finally, when the pressure becomes equal to about one-millionth of an
atmosphere, a luminous glow surrounds the cathode or negative rod,
beyond this is dark space almost filling the bulb, and the walls of the
bulb between the cathode and the anode glow with phosphorescent light.
This phosphorescence is produced by rays coming from the cathode and
passing through the dark space, and these rays have been given the name
of “cathode rays.”
Many interesting experiments with these rays may be performed with
tubes permanently exhausted to the proper degree. The power of the
rays to produce phosphorescence is shown in a most striking way with
a tube fixed in a horizontal position upon a stand, and containing a
light cross made of aluminium, placed in the path of the rays. This
is hinged at the base, so that it can be stood up on end or thrown
down by jerking the tube. Some of the rays streaming from the cathode
are intercepted by the cross, while others pass by it and reach the
other end of the tube. The result is that a black shadow of the cross
is thrown on the glass, sharply contrasted with those parts of the
tube reached by the rays, and which phosphoresce brilliantly. After a
little while this brilliance decreases, for the glass becomes fatigued,
and loses to a considerable extent its power of phosphorescing. If
now the cross is jerked down, the rays reach the portions of the tube
before protected by the cross, and this glass, being quite fresh,
phosphoresces with full brilliance. The black cross now suddenly
becomes brilliantly illuminated, while the tired glass is dark in
comparison. If the tired glass is allowed to rest for a while it partly
recovers its phosphorescing powers, but it never regains its first
brilliance.
An even more striking experiment may be made with a horizontal tube
containing a tiny wheel with vanes of mica, something like a miniature
water-wheel, mounted on glass rails. When the discharges are sent
through the tube, the cathode rays strike against the vanes and cause
the little wheel to move forward in the direction of the anode. Other
experiments show that the cathode rays have great heating power, and
that they are deflected by a magnet held close to the tube.
For a long time the nature of these cathode rays was in dispute. German
physicists held that they were of the same character as ordinary light,
while English scientists, headed by Sir William Crookes, maintained
that they were streams of extremely minute particles of matter in a
peculiar fourth state. That is to say, the matter was not liquid, or
solid, or gaseous in the ordinary sense, but was _ultra-gaseous_,
and Crookes gave it the name of _radiant matter_. Most of us have
been taught to look upon the atom as the smallest possible division
of matter, but recent researches have made it clear that the atom
itself is divisible. It is believed that an atom is made up of very
much more minute particles called _electrons_, which are moving
about or revolving all the time with incredible rapidity. According
to Sir Oliver Lodge, if we imagine an atom of hydrogen to be as big
as an ordinary church, then the electrons which constitute it will
be represented by about 700 grains of sand, 350 being positively
electrified and 350 negatively electrified. It is not yet definitely
determined whether these electrons are minute particles of matter
charged with electricity, or whether they are actually atoms of
electricity. The majority of scientists now believe that the cathode
rays consist of a stream of negative electrons repelled from the
cathode at a speed of 124 miles per second, or not quite 1/1000 of the
velocity of light.
In November 1895, Professor Röntgen, a German physicist, announced
his discovery of certain invisible rays which were produced at the
same time as the cathode rays, and which could penetrate easily solids
quite opaque to ordinary light. He was experimenting with vacuum tubes,
and he found that certain rays emerged from the tube. These were not
cathode rays, because they were able to pass through the glass, and
were not deflected by a magnet. To these strange rays he gave the name
of the “_X_,” or unknown rays, but they are very frequently referred to
by the name of their discoverer.
It was soon found that the Röntgen rays affected an ordinary
photographic plate wrapped up in black paper so as to exclude all
ordinary light, and that they passed through flesh much more easily
than through bone. This fact makes it possible to obtain what we may
call “shadow-graphs” of the bones through the flesh, and the value of
this to the medical profession was realized at once. The rays also
were found to cause certain chemical compounds to become luminous. A
cardboard screen covered with one of these compounds is quite opaque to
ordinary light, but if it is examined when the Röntgen rays are falling
upon it, it is seen to be brightly illuminated, and if the hand is held
between the screen and the rays the bones become clearly visible.
[Illustration: FIG. 38.--X-Ray Tube, showing paths of Cathode and
X-Rays.]
Röntgen rays are produced when the cathode rays fall upon, and as it
were bombard, an obstacle of some kind. Almost any tube producing
cathode rays will produce also Röntgen rays, but special forms of
tube are used when the main object is to obtain these latter rays.
Fig. 38 shows a typical form of simple X-ray tube. This, like all
other tubes for X-ray work, is exhausted to a rather higher vacuum
than tubes intended for the production of cathode rays only. The
cathode C is made of aluminium, and is shaped like a saucer, its
curvature being arranged so that the cathode rays are focused on to the
anti-cathode A. The focusing as a rule is not done very accurately,
for although sharper radiographs are obtained when the cathode rays
converge exactly to a point on the anti-cathode, the heating effect
at this point is so great that a hole is quickly burned. The target,
or surface of the anti-cathode, is made of some metal having an
extremely high melting-point, such as platinum, iridium, or tungsten.
It has a flat surface inclined at an angle of about 45°, so that the
rays emanating from it proceed in the direction shown by the dotted
lines in the figure. The continuous lines show the direction of the
cathode rays. The anode is made of aluminium, and it is shown at N. It
is not necessary to have a separate anode, and the anti-cathode may
be used as the anode. In the tube shown in Fig. 38 the anode and the
anti-cathode are joined by an insulated wire, so that they both act as
anodes. The tube is made of soda-glass, as the X-rays do not pass at
all readily through lead-glass.
[Illustration:
_By permission of_] [_C. H. F. Muller._
FIG. 39.--Diagram of Mica Vacuum Regulator for X-Ray Tubes.]
The penetrating power of the X-rays varies with the vacuum of the
tube, a low vacuum giving rays of small penetration, and a high vacuum
rays of great penetration. Tubes are called hard or soft according to
the degree of the vacuum, a hard tube having a high vacuum and a soft
tube a low one. It should be remembered that the terms high and low,
as applied to the vacuum of X-ray tubes, are only relative, because
the vacuum must be very high to admit of the production of X-rays at
all. The vacuum becomes higher as the tube is used, and after a while
it becomes so high that the tube is practically useless, for the
penetrating power of the rays is then so great that sharp contrasts
between different substances, such as flesh and bone, cannot be
obtained, and the resulting radiographs are flat and poor. The vacuum
of a hard tube may be lowered temporarily by gently heating the tube,
but this is not a very convenient or satisfactory process, and tubes
are now made with special arrangements for lowering the vacuum when
necessary. There are several vacuum-regulating devices, and Fig. 39
is a diagram of the “Standard” mica regulator used in most of the
well-known “Muller” X-ray tubes. This consists of a small additional
bulb containing an electrode D carrying a series of mica discs. A wire
DF is attached to D by means of a hinged cap. The vacuum is lowered
while the discharges are passing through the tube. The wire DF is moved
towards the cathode terminal B, and kept there for a few seconds.
Sparks pass between F and B, and the current is now passing through the
electrode D in the regulator chamber. This causes the mica to become
heated, so that it gives off a small quantity of gas, which passes
into the main tube and so lowers the vacuum. The wire DF is then moved
well away from B, and after a few hours’ rest the tube, now of normal
hardness, is ready for further use.
We have already referred to the heating of the anti-cathode caused by
the bombardment of the cathode rays. Even if these rays are not focused
very sharply, the anti-cathode of an ordinary tube becomes dangerously
hot if the tube is run continuously for a fairly long period, and for
hospital and other medical work on an extensive scale special tubes
with water-cooled anti-cathodes are used. These tubes have a small
bulb blown in the anti-cathode neck. This bulb is filled with water,
which passes down a tube to the back of the target of the anti-cathode.
By this arrangement the heat generated in the target is absorbed by
the water, so that the temperature of the target can become only very
slightly higher then 212° F., which is the temperature of boiling
water, and quite a safe temperature for the anti-cathode. In some tubes
the rise in temperature is made slower by the use of broken bits of ice
in place of water. Fig. 40 shows a Muller water-cooled tube, and Fig.
41 explains clearly the parts of an X-ray tube and their names.
[Illustration: FIG. 40.--Muller Water-cooled X-Ray Tube.]
[Illustration:
_By permission of_] [_C. H. F. Muller._
FIG. 41.--Diagram showing parts of X-Ray Tube.]
An induction coil is generally used to supply the high-tension
electricity required for the production of the Röntgen rays. For
amateur or experimental purposes a coil giving continuous 4-inch or
even 3-inch sparks will do, but for medical work, in which it is
necessary to take radiographs with very short exposures, coils giving
sparks of 10, 12, or more inches in length are employed. An electrical
influence machine, such as the Wimshurst, may be used instead of an
induction coil. Very powerful machines with several pairs of plates
of large diameter, and driven by an electric motor, are in regular
use for X-ray work in the United States, but in this country they are
used only to a very small extent. A Wimshurst machine is particularly
suitable for amateur work. If a screen is to be used for viewing bones
through the flesh a fairly large machine is required, but for screen
examination of such objects as coins in a box, or spectacles in a
case, and for taking radiographs of these and other similar objects, a
machine giving a fairly rapid succession of sparks as short as 2 inches
can be used. Of course the exposure required for taking radiographs
with a machine as small as this are very long, but as the objects are
inanimate this does not matter very much.
For amateur X-ray work the arrangement of the apparatus is simple.
The tube is held in the required position by means of a wooden clamp
attached to a stand in such a way that it is easily adjustable.
Insulated wires are led from the coil or from the Wimshurst machine
to the tube, the positive wire being connected to the anode, and the
negative wire to the cathode. With a small Wimshurst machine light
brass chains may be used instead of wires, and these have the advantage
of being easier to manipulate. For medical purposes the arrangements
are more complicated, and generally a special room is set apart for
X-ray work.
If the connexions have been made correctly, then on starting the coil
or the machine the tube lights up. The bulb appears to be sharply
divided into two parts, the part in front of the anti-cathode glowing
with a beautiful greenish-yellow light, while the part behind the
anti-cathode is dark, except for lighter patches close to the anode.
The Röntgen rays are now being produced. The illumination is not steady
like that of an electric lamp, but it consists of a series of flickers,
which, with powerful apparatus, follow one another so rapidly as to
give the impression of continuity. If the connexions are wrong, so that
the negative wire goes to the anode instead of to the cathode, the bulb
is not divided in this way, but has patches of light almost all over.
As soon as this appearance is seen the apparatus must be stopped and
the connexions reversed, for the tube is quickly damaged by passing the
discharge through it in the wrong direction.
Having produced the X-rays, we will suppose that it is desired to
examine the bones of the hand. For this purpose a fluorescent screen is
required. This consists of a sheet of white cardboard coated usually
with crystals of barium platino-cyanide. In order to shut out all light
but that produced by the rays, the cardboard is placed at the larger
end of a box or bellows shaped like a pyramid. This pyramid is brought
close to the X-ray tube, with its smaller end held close to the eyes,
and the hand is placed against the outer side of the cardboard sheet.
The outline of the hand is then seen as a light shadow, and the very
much blacker shadow of the bones is clearly visible. For screen work
it is necessary to darken the room almost entirely, on account of the
feebleness of the illumination of the screen.
If a radiograph of the bones of the hand is to be taken, a very
sensitive photographic plate is necessary. An ordinary extra-rapid
plate will do fairly well, but for the best work plates made specially
for the purpose are used. The emulsion of an ordinary photographic
plate is only partially opaque to the X-rays, so that while some of
the rays are stopped by it, others pass straight through. The silver
bromide in the emulsion is affected only by those rays which are
stopped, so that the energy of the rays which pass through the emulsion
is wasted. If a plate is coated with a very thick film, a larger
proportion of the rays can be stopped, and many X-ray plates differ
from photographic plates only in the thickness of the emulsion. A thick
film however is undesirable because it makes the after processes of
developing, fixing, and washing very prolonged. In the “Wratten” X-ray
plate the emulsion is made highly opaque to the rays in a different and
ingenious manner. Salts of certain metals have the power of stopping
the X-rays, and in this plate a metallic salt of this kind is contained
in the emulsion. The film produced in this way stops a far larger
proportion of the rays than any ordinary film, and consequently the
plate is more sensitive to the rays, so that shorter exposures can be
given.
X-ray plates are sold usually wrapped up separately in light-tight
envelopes of black paper, upon which the film side of the plate is
marked. If there is no such wrapping the plate must be placed in a
light-tight envelope, with its film facing that side of the envelope
which has no folds. The ordinary photographic double envelopes, the
inner one of yellow paper and the outer one of black paper, are very
convenient for this purpose. The plate in its envelope is then laid
flat on the table, film side upwards, and the X-ray tube is clamped in
a horizontal position so that the anti-cathode is over and pointing
towards the plate. The hand is laid flat on the envelope, and the coil
or machine is set working. The exposure required varies so much with
the size of the machine or coil, the distance between the tube and the
plate, the condition of the tube, and the nature of the object, that it
is impossible to give any definite times, and these have to be found
by experiment. The hand requires a shorter exposure than any other
part of the body. If we call the correct exposure for the hand 1, then
the exposures for other parts of the body would be approximately 3 for
the foot and the elbow, 6 for the shoulder, 8 for the thorax, 10 for
the spine and the hip, and about 12 for the head. The exposures for
such objects as coins in a box are much less than for the hand. After
exposure, the plate is developed, fixed, and washed just as in ordinary
photography. Plate XIV. shows a Röntgen ray photograph of a number of
fountain pens, British and foreign.
Prolonged exposure to the X-rays gives rise to a painful and serious
disease known as X-ray dermatitis. This danger was not realized by
the early experimenters, and many of them contracted the disease,
with fatal results in one or two cases. Operators now take ample
precautions to protect themselves from the rays. The tubes are screened
by substances opaque to the rays, so that these emerge only where they
are required, and impenetrable gloves or hand-shields, aprons, and
face-masks made of rubber impregnated with lead-salts are worn.
X-ray work is a most fascinating pursuit, and it can be recommended
strongly to amateurs interested in electricity. There is nothing
particularly difficult about it, and complete outfits can be obtained
at extremely low prices, although it is best to get the most powerful
Wimshurst machine or induction coil that can be afforded. As
radiography is most likely to be taken up by photographers, it may be
well to state here that any photographic plates or papers left in
their usual wrappings in the room in which X-rays are being produced
are almost certain to be spoiled, and they should be placed in a
tightly fitting metal box or be taken into the next room. It is not
necessary for the amateur doing only occasional X-ray work with small
apparatus to take any of the precautions mentioned in the previous
paragraph, for there is not the slightest danger in such work.
[Illustration: PLATE XIV.
_By permission of_ _Kodak Ltd._
RÖNTGEN RAY PHOTOGRAPH OF BRITISH AND FOREIGN FOUNTAIN PENS. TAKEN ON
WRATTEN X-RAY PLATE.]