How the X-ray Machine was Discovered and Works: Hertz & Lenard

On November 8th of 1895, a man named Wilhelm Roentgen turned on a covered vacuum tube and accidentally discovered the x-ray which changed our world.  This is not that story.   Instead, this video is about how (and why) Roentgen had an x-ray machine in the first place. The x-ray machine was not made to create x-rays as they didn’t know x-rays existed.  In fact, the x-ray machine had been accidentally invented 16 years earlier but no one noticed x-rays until a man named Lenard covered a tube the year before.  So, how do tubes create x-rays and why in the world would anyone cover it and why was that important?  Ready for the story? Let’s go!

I am going to start in 1882 when a 24-year-old German Physicist named Heinrich Hertz wrote his parents, “I am busy from morn to night with optical phenomena in rarefied gases[i].”  Hertz, like almost all of the scientific community, was inspired by the experiments and conclusions of a charismatic English chemist named William Crookes.  Four years earlier, Crookes demonstrated that if he applied a high voltage across a glass tube with a high vacuum (or though a gas that was rarefied) an invisible “cathode ray” would emanate from the negative electrode and make the glass on the other end glow.  He wasn’t the first to do this, but due to some clever experiments, he was the first to think that this ray was a beam of charged atoms!  This was a radical idea and soon everyone was playing with “Crookes tubes”.  Hertz found that although the cathode ray would bend with a magnetic field it would not move with an electric field and decided that cathode rays were a type of ultraviolet light, concluding, that cathode rays “have no closer relation to electricity than the light emanating from an electric lamp. [ii]”  [By the way, Hertz didn’t realize that trace amounts of gas were insulating the rays from electric fields, and, years later JJ Thompson figured that out and then determined that these rays are not UV light or charged atoms but actually charged sub-atomic particles called electrons!]

Heinrich Hertz
Heinrich Hertz

Hertz then moved from electrifying tubes to electrifying an antenna, which is how, in 1886, he discovered the radio wave.  By the way, radio is an invisible electromagnetic wave that has “no closer relation to electricity than the light emanating from an electric lamp”!  Crazy huh?  In December of 1890, Hertz went to England to receive the Rumford medal for his work on radio waves where he met Crookes.  Perhaps, for this reason, Hertz went back to studying the Crookes tube.  This time he wondered about the power and heat from the rays.  Crookes had melted a piece of platinum foil by placing the foil in the middle of the tube and focusing the cathode ray on it.  Hertz, however, used an unfocused cathode ray, put the foil on the end of the tube against the glass, and used gold as it was easier to melt.  To his surprise, the gold didn’t melt, but it also didn’t block the path of the cathode rays!  The gold leaf “looks like a faint veil upon the glass plate, chiefly recognizable at its edges and by the slight wrinkles in it.[iii]”  This seemed to verify his theory that the cathode ray was a wave, as he thought particles couldn’t travel through solids without breaking the solid [he was wrong].  Hertz then tried many different materials from silver to platinum and copper and found they all worked but noted that aluminum seemed to be “the most suitable for the experiments,” as the light was blocked by it, and cathode rays were not blocked and it was so easy to use.  Tragically, mere months after this publication, he started having severe headaches and quit researching.  He died in 1894 when he was just 36 years old from blood poisoning.

Before Hertz gave up on research, he called his assistant, Phillip von Lenard, and said, “we ought… to separate two chambers with aluminum leaf and produce the rays as usual in one of the chambers…[and] observe the rays in the other chamber.[iv]” Now Lenard had been fascinated with cathode rays ever since he had heard about Crookes talks.  Also years earlier Lenard had read Hertz’s old paper that “proved” that cathode rays were ultraviolet light and had an ingenious idea.  Lenard knew that quartz crystals transmit ultraviolet light so he made a tube with a quartz “window” with the hope that the cathode rays would escape the tube through the quartz.  As cathode rays are beams of electrons this did not work.  After talking to Hertz, Lenard now knew of an object that would transmit cathode rays – aluminum. Lenard recalled that he took, “the old tube again, I replaced the quartz with a metal plate containing a small hole sealed with aluminum foil, spread a few small grains of alkaline earth phosphor on this small aluminum window, excited the tube and, lo and behold, the grains glowed brightly!”  The cathode rays had escaped to the outside of the tube!  Astonishingly, (and Lenard said, “nobody could have predicted this”), the cathode rays could not only escape the tube they could also travel a few inches in the air.  Lenard was ecstatic, but it was also, “a breakthrough into the unknown.”  He then tried everything he could do to increase the intensity of the beam, which he accomplished by using the highest voltage he could.  Lenard didn’t know it, but he had just made an x-ray machine that was perfectly situated to discover the x-ray.  Years later, Lenard complained, “All Roentgen had to do was push a button, since all the groundwork had been prepared by me.”[v] 

Now, you should know that Lenard’s tube was actually not very good at creating x-rays and the Crooke’s tubes made 15 years earlier where he melted platinum was a far superior x-ray machine.  However, as Crookes was busy studying the platinum he never realized what he was creating.  He even accidentally developed some of his photographic plates from his x-ray machine, but just thought the plates were defective and returned them to their manufacturer![vi]  Over the years several other people (notably Arthur Goodspeed in 1890, Fernando Sanford in 1891, and Nikola Tesla in 1894) had accidentally taken x-ray pictures with vacuum tubes although none of them realized exactly what they were looking at.  What made Lenard’s tube perfect for detecting x-rays because it was the first that you studied from the outside.  Inside the tube, it was very difficult to distinguish between fluorescence due to the cathode ray and the x-rays.  Outside Lenard’s tube, the only thing that was supposed to emanate were cathode rays through the aluminum “window” and they only travel a couple of inches in the air!  Therefore, all you needed to discover x-rays was a covered Lenard tube in a dark room, good vision, an appropriate fluorescent screen, and the luck to notice glow feet away from the side of the tube.  If you are wondering why Lenard didn’t notice x-rays first it was because he was using a type of fluorescent screen that was mostly composed of “light” elements with low atomic numbers, which are mostly transparent to x-rays (but work fine with cathode rays).  Roentgen happened to use a different screen that was about as efficient at detecting cathode rays but contained a lot of “heavy” elements that were “approximately 100 times more efficient for the detection of x-rays.[vii]

Crookes tube

So, how did Lenard’s tube make x-rays and why was the old Crooke’s tube where he melted platinum superior?  In both Lenard’s tube and Crooke’s tube (and in all x-ray machines), a beam of electrons hits a solid at a fast speed.  This is because when a high voltage was placed across a vacuum tube, negative electrons on or near the cathode are repelled from the negative side and speed towards the other side.  When the electrons hit the solid, they can create x-rays in two ways.  First, electrons can be diverted by the nucleus of the atoms and as they whip around the atom they lose energy and produce x-rays (called bremsstrahlung – for braking radiation).  If the atom is “light” meaning having few protons (like Aluminum) then it produces less breaking so it produces less breaking radiation but if the atom is “heavy” like Platinum it creates more breaking radiation.  Second, the incoming electrons can actually knock out a low-energy electron.  When this happens an electron in a higher shell “falls” down to fill in the empty spot and releases that change in energy as an x-ray (called x-ray fluorescence).  This produces the more powerful x-rays.  However, with “light” atoms these x-rays are not emitted as they tend to be reabsorbed by the same atom to make it release another electron instead (called an Auger electron).  In conclusion, aluminum makes less powerful bremsstrahlung x-rays and almost no fluorescence x-rays.  This is why x-rays were first discovered with covered Lenard tube but Crooke’s platinum tubes were quickly found to make more x-rays and were used for x-ray machines for the next 20 years or so (until they learned to heat the cathode to make it easier to create an electron beam and cool the target with water).   

It is easy to dismiss Lenard’s contribution because he turned into this big Nazi (and pain in both Roentgen’s and then Einstein’s side), but he really did have an important role to play in the history of x-rays (and of physics in general).  This is not, in any way, to diminish what Roentgen accomplished.  Roentgen didn’t just notice x-rays, but, by himself with primitive equipment and over the course of just weeks, Roentgen named x-rays, determined that x-rays are an electromagnetic wave, that they come from the cathode ray hitting the wall, that they are blocked by lead, and even took the first medical x-ray (of his wife’s hand).  Once he took the x-ray of his wife’s hand, according to Roentgen, “all hell broke loose[viii]”.  And that story is next time on the Lightning Tamers. 


[i] Hertz, Heinrich (in a letter to parents dated June 28, 1882) Hertz, Heinrich Miscellaneous Papers with an introduction by Philipp Lenard, 1895 p. xxi

[ii] Hertz, Heinrich “Glow Discharge Experiments“Annalen der Physik und Chemie, Vol. XIX 1883 p. 814

[iii] Hertz, Heinrich “About the Passage of the Cathode Rays Through Thin Metal Layers” Annalen der Physik, vol. 45 p 29

[iv] As quoted by Philipp Lenard in his Award Ceremony Speech, Nobel Prize, 1905 p.  107

[v] referenced in Hilman, Bruce The Man Who Stalked Einstein 2015 p. 77

[vi] Seliger, Howard “Wilhelm Conrad Röntgen and the Glimmer of Light” Physics Today Nov. 1995 p. 25

[vii] Seliger, Howard “Wilhelm Conrad Röntgen and the Glimmer of Light” Physics Today Nov. 1995 p. 28

[viii] referenced in Dibener, Bern The New Rays of Rofessor Roentgen, 1963 p. 23

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