The Origin of the Cathode Ray Tube: Physics of the Geissler tube to the Crookes tube

Let’s start in early 1857 in a laboratory in Bonn, Germany.  That is where a physics professor named Julius Plücker was working with an instrument maker named Heinrich Geissler to create odd glass tubes that were, according to Plücker, “incomparably beautiful[i].” 

Geissler and Plücker didn’t know it, but these Geissler tubes would change the world.  These were the precursor to cathode ray tubes (also called Crookes tubes) and without them you wouldn’t have television, the oscilloscope, or even x-rays.  Moreover, it was with the cathode ray tube that the electron was discovered!

So, what were Geissler tubes, how did they work, why were they invented and how were they changed by his shy assistant and a man with a fabulous mustache into the cathode ray tube?

Table of Contents

Geissler Tubes

How a Geissler Tube Works

The Difference between Plucker and Geissler

Hittorf’s Discovery

William Crookes

Crookes’ Tube


Geissler Tubes

Geissler tubes started pretty simply, they were skinny glass tubes with metal electrodes (platinum wires) at either end.  The tubes were mostly evacuated (had air removed with a pump) and were filled with trace amounts of certain gasses or vapors.  Geissler had just invented a mercury pump and his friend Rumhkorff was selling a device called an induction coil that would produce very high voltages so they had all of the equipment needed to do this experiment. 

How a Geissler Tube Works

But how did it work?  Well, when a high voltage is placed across the tube, some electrons are ripped free of their atoms (leaving positive ions, or atoms missing an electron) and the electrons go zipping towards the positive terminal.  On the way, they recombine with the ions creating visible light. 

The color produced depends on the energy levels of the gas or vapor molecules in the tube, which is why different gasses produce different colors.  [You need to have low pressure because if the pressure is too high the electrons don’t get very much kinetic energy before they bump into an ion, which means that they won’t give the gas enough energy to produce light]

Now scientists had been making sparks in evacuated chambers for well over 100 years at this point, in an experiment called “the electric egg”.  In fact, just a few years earlier Rumhkorff and his friend Quet had used the spark from Ruhmkorff’s induction coil in an “electrical egg” and had even added various gasses or vapors. 

The Difference between Plucker and Geissler

What made Plücker and Geissler’s experiment different is that they were not trying to create a spark in a vacuum or through gasses; they were trying (and succeeding) in using the voltage to electrify the gasses directly. Although Plücker was pleased with the beautiful lights that they were making he was mostly interested in how the light interacted with magnets, which is why his paper on the tubes was titled,

“About the Influence of the Magnet on the Electrical Discharges in Diluted Gases” (catchy eh?).  Geissler, however, saw the financial angle and started a business selling increasingly convoluted and beautiful glass tubes to the public as novelty items.

Now Plücker had a shy graduate student named Johann Hittorf who, a full ten years later, noticed that Geissler’s vacuum pump would bubble for hours from an area that was supposed to be a vacuum.  He deduced that the surfaces had moisture in it that were interfering with the efficiency of the pump so he removed it with a chemical reaction and improved Geissler’s pump considerably. 

Hittorf’s Discovery

Hittorf then immediately used this adjusted pump on a Geissler tube and found a strange result.  It seemed like there was light coming from the negative electrode and if he made the electrode into a sharp point, this light would hit the far glass and make it glow, either green or blue depending on the type of glass.

He proved that the light was coming from the negative electrode by making a special L shaped tube and noting that the glass glowed across from the negative electrode.  He also found that if he placed an object in its way it would cast “a sharp shadow”.  Hittorf called these “rays of glow” and deduced that “any point of the cathode is the source of a cone of rays”. 

[Side note, Faraday defined the cathode as where the electricity came from, so as the rays of glow came from the negative electrode, the negative electrode was called the cathode and the ray was renamed a cathode ray]. What was going on?  Electrons from the negative electrode and from near it were repelled by it and a few bumped into ions and created a glow. 

However, there were less molecules so many electrons made it all the way to the far glass with enough energy to cause them to glow with a color depending on the type of glass.  However, because this “light” would cause the glass to fluoresce, Hittorf assumed the cathode ray was just a new type of Ultraviolet light.  Unfortunately, Hittorf was reserved and not a good speaker and few people knew or were interested in his results.   

William Crookes

Now we go to an English scientist named William Crookes.  In later life, Crookes’s most distinguishable feature was his completely amazingly over the top mustache!  Here is a caricature of him and here is a photo!  Anyway, Crookes came from a truly enormous middle class family (his father had five children with his first wife and, ready for it, 16 children with his second wife!). 

Crookes always liked science, and recalled he spent his childhood, “reading any book of science I could find [and]…generating smells and destroying furniture” although most of his family was uninterested, “I don’t suppose any of my family even knew the meaning of the word ‘science’ and I was always regarded as a bit of a fool.

[1]” Despite his families distaste, he went to college and became a Chemist.  In 1859 or 1860, Crookes learned about a new way of studying chemicals by the light it created (called spectroscopy) and used it to discover a new element he called thallium (for green twig).  He then became fascinated with the relationship between temperature, light, and molecular theory. 

Crookes’ Tube

In 1869, a talented 16 yr. old named Charles Gimingham became Crookes assistant and by 1876 created a vastly superior vacuum pump for his boss.  Crookes then used this new pump on the Geissler tube and found very similar results to Hittorf, although at the higher vacuums the tube itself seemed black and only the glass at the end would glow. 

In addition, he moved the positive anode to the side to get it out of the way.  Unlike Hittorf, Crookes was a fantastic speaker [after a talk on the cathode ray tube, a friend wrote him that his “wonderful experiments” were “discussed at a thousand breakfasts this morning”].

Also, unlike Hittorf, Crookes had a vastly different idea of what was going on inside the tubes, he didn’t think that they were beams of ultraviolet light, he thought they were beams of charged particles that could cause items to fluoresce! After all, as Plücker had noticed with the Geissler tubes, the position of the rays could be moved with a magnet and magnets were known to move current carrying wires but do not move beams of light. 

In addition, Crookes built a tube with a little wheel inside that would spin away from the cathode and towards the anode, which Crookes attributed to “molecular pressure”.In 1879 he wrote, “we seem at length to have within our grasp and obedient to our control the little indivisible particles which constitute the physical basis of the Universe”. 

What Crookes missed, however, that this “radiant matter” was not the “forth state of matter” with it’s own rules.  Instead, he was on the cusp of a deeper result.  For “cathode rays” are really beams of electrons that are not a different state of matter or a different type of molecule, they are a building block of matter.  Crookes had discovered a subatomic particle (meaning smaller than atomic).  However, Crookes couldn’t figure out how to prove that his rays were particles let alone what their mass or charge was.

[On the other hand, BECAUSE of cathode rays Crookes actually predicted the idea of plasma, photons, curved spacetime, absolute zero temperature, and that mass was related to energy all back in 1879, the year Einstein was born!  No wonder his friend Alice Bird told him that he seemed,“like the magician of the Future before whom no secrets are hid.”]

Back in Germany, Heinrich Hertz (who had used the Ruhmkorff coil to discover radio) was sure that Crookes was wrong and that Hitorff was correct and cathode rays are a beam of electromagnetic waves.  In 1892, Hertz created an experiment with a piece of gold that he thought would prove him right.  Hertz’s results weren’t as convincing as he thought but instead it led directly to the invention of the x-ray machine.  And that story is next time on “The Lightning Tamers”

I don’t have a video on how Rumhkorff invented his induction coils because he didn’t invent them, he just produced and sold ones of very high quality in the mid 1800s.  In fact, he even was given a 50,000 franc prize in 1858 for creating “the most important discovery in the application of electricity”!  (Shows what good marketing can do).  But if you are interested in how induction coils were invented please watch my video on the history of the Tesla coil. 

1879“The phenomena in these exhausted tubes reveal to physical science a new world – a world where matter exists in a fourth state, where the corpuscular theory of light holds good, and where light does not always move in a straight line”  “matter is but a mode of motion; at the absolute zero of temperature the inter-molecular movement would stop, and… matter, as we know it, would cease to exist.  Believe me, …William Crookes”  Crookes, William, “On a forth state of matter” Proceedings of the Royal Society of London, published January 1, 1879 p. 470

Crookes “the molecular ray which gives birth to green light absolutely refuses to turn a corner, and radiates from the negative pole in straight lines, casting strong and sharply-defined shadows of anything which happens to be in its path. On the other hand, the ordinary luminescence of vacuum tubes will travel hither and thither along any number of curves and angles.”


[1] Plücker “Ueber die Einwirkung des Magneten…” Annalen der Physik und Chemie vol. 103 (1858) p. 88

[1]d’Albe, Fournier The Life of Sir William Crookes p. 16

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