October 1960 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
A while back I was taken to task for remarking on the seemingly
crude (I think the word I used was 'barbaric") nature of some early
medical machinery that appeared in a mid-20th-century edition of
Popular Electronics (see "Electronics
Against Cancer"). I won't repeat my mistake here when describing
how early medical x-ray machines are scary looking. Admittedly,
being stuffed inside a modern MRI machine and having the loud electromagnetic
coils cycling is no relaxing trip to the countryside, but the x-ray
machine shown here looks like the Q-36 space modulator beam Marvin
the Martian might have pointed at the Earth to disintegrate it just
before he asks, "Where's the kaboom?" The anxiety caused by the
behemoth's presence could not have given much comfort to the
poor souls being treated. Seriously, though, there is a lot of good
info about the evolution of lasers in this article.
After Class: X-Rays
By Fred E. Ebel, W9PXA
by chance only 65 years ago, X rays are one of our most valuable
Is it light?
Is it electricity?
Not in any known form.
What is it?
I don't know.
With such scientific frankness did physics professor Wilhelm
Konrad Roentgen relate his discovery of a mysterious new ray to
a newspaper reporter. Stumbled upon by accident during a routine
laboratory experiment on the night of November 8, 1895, the new
ray was dubbed an "X ray" by Roentgen. "X," then as now, was the
mathematical symbol for the unknown.
Much of the "X" has been taken out of X rays since the modest
German physics professor demonstrated this startling radiation and
its power to penetrate tin, paper, wood, and even the human body.
When X-rays were first put into use, you ordinarily had to visit
a hospital to see them in action. Today, unlimited industrial applications
make X rays far more than just a diagnostic and therapeutic tool
of medical science.
- or Fate? Just what did happen on the night of November
8, 1895? Call it fate, fortune, or chance, but there were a number
of conditions that conspired to make this night one to remember.
First, Roentgen had completely covered the Crookes tube he was using
with a black cardboard, making it light-tight. Secondly, his laboratory
itself was plunged in darkness. Finally, the piece de resistance
- a sheet of paper painted with crystals of barium platinocyanide
- lay on a bench some distance from the tube.
The barium platinocyanide screen was the "chance" that nature
gave Roentgen to unlock one of her secrets. For when the crystals
glowed with a shimmering yellow-green fluorescence, Roentgen's keen
scientific mind became curious. True, cathode rays could make the
crystals glow - but at this distance? He placed the crystal screen
at an even greater distance from the tube than the range cathode
rays were known to penetrate. Still the strange fluorescence!
Heart pounding, he grabbed a book and placed it between the Crookes
tube and the screen. The crystals continued to glow. Whatever it
was, it was coming through the book! Next, he tried metals - and
found that the rays penetrated in varying degrees, although lead
and platinum stopped them completely.
Now came the most dramatic test of all.
exposed his hand, and - his heart must have almost stopped - saw
the shadows of his bones. As Roentgen made photographs of his findings,
it was obvious that he had found something far more exciting than
cathode rays - X rays!
How X Rays Are Formed. How was Roentgen able
to produce these powerful rays with his crude apparatus - a modified
Crookes tube, mercury interrupter, and a Ruhrnkorff induction coil
that furnished a bare 20,000 volts? The simple fact is this: X rays
are relatively easy to generate. You simply speed up electrons and
let them collide with a target. The electrons cause disturbances
within the atoms of the target, releasing X rays. Any material,
even a gas or liquid, will release X rays when bombarded by high-velocity
Obviously, then, glass can be a target, as is was in the Crookes
tube Roentgen used - see Fig. 1. Here a gas-type tube, consisting
of an anode and cold cathode, was connected to a high-tension induction
coil. Heavy positive ions in the residual gas were drawn to the
negative cathode (unlike charges attract), striking with such force
that they knocked electrons from the cathode metal. It was this
positive-ion bombardment that created and maintained a source of
electrons, the "work horses" for X-ray generation.
The negatively charged electrons, in turn, were drawn toward
the high-voltage positive anode. The resultant stream of electrons,
actually cathode rays, traveled so fast - about 30,000 miles per
second - that most of them could not "turn the corner" to reach
the anode. Instead, they smashed into the glass wall of the tube.
The glass, therefore, was the target, providing the barrier for
the sudden stoppage of electrons. The result: radiation of X rays,
and, of course, the glow of fluorescence of the glass that Roentgen
Nature of X Rays. X rays are electromagnetic rays
similar to visible light rays, with this important exception - their
wavelength is very small, about 1/10,000th that of light. These
tiny wavelengths are measured in angstroms, units so small that
you can line up 254,000,000 of them between the one-inch marks on
a ruler. The X-ray region in the electromagnetic spectrum ranges
from about 0.006 to 1000 angstrom units. Interestingly enough, it
is this exceedingly short wavelength of X rays that makes possible
their penetration of matter, and which enables the researcher to
delve into the vast voids of molecular inner space.
Fig. 1. - Crookes tube used by Roentgen produced X rays
when electrons flowing from its cathode to its anode bombarded
the glass tube.
Fig. 2. - Basic X-ray unit includes X-ray tube and high-voltage
Fig. 3. - X-ray tube detail. The focusing cup concentrates
electron stream from cathode and directs it toward tungsten
Unlike the cathode ray generated in your TV picture tube, X rays
are non-electrical. Thus, they are unaffected by electrostatic and
magnetic fields. This can be proved by placing a magnet or charged
plate near X rays; they "ill be neither attracted nor repelled as
in the case of cathode rays.
Traveling at e same speed as light and radio waves - 186,000
miles per second, X rays can be reflected and refracted only at
very small angles. (Roentgen failed to focus X rays, despite many
experiments with lenses of wood, glass, aluminum, and other materials,
for this reason.)
The darkening of photographic film by X rays has given them wide
application in medicine, research, and industry. A radiograph used
by makers of cast-metal products is actually a shadow picture of
the subject. The dark regions of the film represent the more penetrable
parts - gas pockets in a weld, for example; the lighter regions
identify the more opaque areas.
How X Rays Work. A basic X-ray unit is comprised
of filament, high-voltage transformer and timing circuits - see
Fig. 2. The heart of the unit is the X-ray tube. Like Roentgen's
original tube, the modern tube also has a cathode and an anode,
but with tremendous improvements. Now the tube is evacuated to an
extremely high vacuum. The cathode structure contains a coil of
tungsten wire - the filament - which "boils off" electrons when
heated to incandescence. A metal reflector or focusing cup on the
cathode directs the electron beam toward the target-as shown in
Tungsten is ordinarily used for the target material, since it
can withstand high temperatures without melting. This is important
because less than 1% of the energy in the electrons is converted
to X rays upon bombardment with the target; most of the energy is
converted to heat. To help dissipate the heat, the tungsten is imbedded
in a large mass of copper which conducts the heat into air or into
oil, as in the case of the oil-immersed tube.
It is desirable to have the focal spot - the area of the target
that receives the electron bombardment - as small as possible. The
smaller the focal spot, the better the detail of the radiograph.
But a small focal spot means an intense blast of electrons in a
tiny area; even tungsten melts under such grueling treatment. This
problem can be solved by simply rotating the anode target. The target
constantly turns another "face" to the electron stream, area - see
An induction motor provides the rotating power in an ingenious
way. The stator surrounds the outside of the evacuated glass bulb
tube and provides the rotating magnetic field that turns the rotor
in the tube at approximately 3000 rpm. The rotor in the "neck" of
the tube is, of course, connected to the target. The entire moving
assembly is located inside the evacuated tube.
The high-voltage circuit consists of a step-up transformer and
its controls; an autotransformer supplies voltage to the primary
winding of the high-voltage transformer. Any change of the autotransformer
voltage produces a corresponding change in the high-voltage output
which is applied to the X-ray tube. Changes in voltage are made
with a selector switch; increasing the tube voltage results in a
decrease in wavelengths of X rays, accompanied by an increase in
If "soft" X rays of low penetrability and longer wavelengths are
desired, the selector switch is set at about 20,000 volts. But if
"hard" X rays of high penetrability and shorter wavelengths are
desired, the switch is set for several hundred thousand volts.
Fig. 4. - Rotating anode in target constantly turns new
face to electron stream in order to distribute heat over
a wide area.
Fig. 5. - High-voltage rectifier circuit used in some X-ray
Rectification of the high-tension alternating current to the
tube can be very simple - in fact, the circuit can be made self-rectifying.
Current will flow through the tube only on the half cycle when the
anode is on its negative half cycle, since the anode now repels
the negative electrons. Some X-ray systems use a high-voltage full-wave
rectifier circuit, permitting conduction of current through the
tube on each half cycle of alternating current-see Fig. 5.
Present Day Uses. Quality control in manufacturing
makes extensive use of the non-destructive quality of X rays in
the inspection of casting and weldments for such defects as cracks
and gas pockets. X-ray devices in beverage plants "look into" opaque
cans moving rapidly on a conveyor line and give the signal for automatic
rejection of under-filled cans.
Similar devices reveal foreign bodies in food stuffs; detect
the hollow heart of potatoes; separate pithy from juicy oranges;
and reveal the improper assembly of electronic tubes, switches,
and small electrical assemblies. X rays also gauge the thickness
of electroplating, as well as that of hot steel strip racing along
at 4000 feet per minute in a rolling mill.
Dramatic applications abound in X-ray diffraction. Here, X rays
are made to bounce off mirror-like atomic planes of crystalline
substances to reveal secrets of inner structure. Nylon, magnetic
TV tape. synthetic rubber, high-temperature alloys, high-test gasolines,
and penicillin are just a few of the products X-ray diffraction
has helped to develop or improve.
Art museums use X rays to examine the authenticity of old paintings.
In other applications, X rays distinguish real diamonds and pearls
from their imitations.
Biologically Speaking. It is now well known
that X rays as well as gamma rays can mutate or change the genes
(hereditary units) of our bodies. Excessive X-radiation can also
affect flesh, bone, and blood destructively. For these reasons,
it is of utmost importance that exposure to radiation be kept at
What can be done in this respect? So far as background radiation
is concerned, even Adam and Eve had to contend with the small amount
of gamma radiation from radioactive material which occurs naturally
in soil, rocks, and even plants. In fact, there are radioelements
in our bodies that give each of us a daily unavoidable radiation
dose of 0.0001 roentgen. (The roentgen is the unit of X- and gamma-ray
dose.) In addition, cosmic rays from interstellar space add to our
daily dose of background radiation.
In essence, X rays are simply a form of man-made radiation, but
new techniques and advancements greatly reduce the effects of their
exposure to patients. Diagnostic voltages now up to 150 kv. permit
much shorter exposure times, as do faster films. Significant, too,
are collimators that confine the X-ray beam to the exact area desired.
All in all, few would deny that the tremendous diagnostic and
therapeutic benefits of X rays far outweigh any possible deleterious
effects. In fact, many a man, woman, and child is alive today because
of Roentgen's startling discovery. Since that eventful night in
1895, these once strange and unknown rays have done much to alter
the nature of the world we live in.
Posted May 20, 2014