Famous Moments in Science (1851-1900)


I am going to try to collect key articles, descriptions, examples, etc. of great moments (e.g. experiments) in the sciences, and in particular in physics (my favourite topic). And I am going to try to avoid lots of equations, etc.

So far my contributions on this page include:

William Crookes (gas-discharge tubes) - I have added an extended discussion on different types of early gas-discharge tubes, including the ones of Geissler and Plücker, their gradual evolution in to cathode ray tubes with Hittorf, Goldstein and Lenard, then in to X-ray tubes with Röntgen, Jackson, and Coolidge. And not forgetting a mention for Braun and the first oscilloscope.

Johann Wilhelm Hittorf (electrolysis, cathode ray tubes)

Eugen Goldstein (cathode ray tubes)


William Crookes (GB, 1832-1919) is certainly best known for a version of the vacuum tube, known as a Crookes tube. In modern day jargon it is a discharge tube in which cathode rays, a stream of electrons, were discovered. It is said that Röntgen discovered X-rays in 1895 using a Crookes tube.

The Crookes tube was a cold cathode tube, meaning that there was no heated filament used to emit electrons. Below we can see an early example, with the glass extension and stopper that allowed for the control of the vacuum and the introduction of different gases. The electrodes (cathode and anode) were connected to a high voltage generator (induction coil or electrostatic machine). The visible cathode beam (electrons) between the electrodes could be varied depending upon the voltages applied, the degree of vacuum, and the nature of the residual gas. We can see that the electrodes were just flat aluminium discs connected to platinum wires sealed into lead glass, which was then fused into a 26 cm long, thin-walled glass cylinder.

In this type of tube the electrons are liberated by ionisation of the residual air (or gas). The gas pressure in the tube was in the region 10-6 to 10-8 atmospheres. A high DC voltage (kV to 100 kV) was then applied between the electrodes. The electric field accelerates the few ions and free electrons always present in the gas (i.e. from photoionisation or radioactivity). These particles collide with other gas molecules, creating more electrons and ions, creating a chain reaction (avalanche multiplication) called a Townsend discharge. Positive ions go to the cathode, and on impact free more electrons from the surface. These electrons stream to the positively charged anode, they are what are called cathode rays.

The tube is under low pressure, and the DC voltage is high. So most of the electrons will be accelerated to the anode, attaining about 20% of the speed of light with a 10 kV applied DC voltage. These electrons (cathode rays) have so much momentum that they pass though the anode and hit the back of the glass tube. The electrons hit the atoms in the glass, knocking orbital electrons into higher energy levels. These electrons fall back to their ground states emitting light. This fluorescence causes the glass to glow yellow-green, and thus reveals the beam of electrons striking the glass. Eventually the electrical charge is collected on the anode and returns to the power supply. As you can imagine the actual modern-day description is more complex because we now consider that the tube contains a non-equilibrium plasma of charge ions, electrons, and neutral atoms.

There is a fantastic site dedicated to the cathode ray tube, which in particular covers induction coils, Crookes and Geissler tubes, X-ray tubes and the later camera tubes and CRT’s.  

As a little aside, many descriptions just mention a DC high voltage, but do not mention how it was obtained (at best an induction or Ruhmkorff coil is mentioned). Heinrich Rühmkorff  (DE, 1803-1877) made induction coils in Paris during the period from 1855 through to when he died (he dropped the ü in Paris), but the induction coil was in fact invented by Nicholas Callan (IE, 1799-1864).

It consisted of two coils of insulated copper wire wound around a common iron core. The primary winding would have several hundreds of turns, the secondary many thousands. A current passing through the primary coil creates a magnetic field, which, with the common core, couples with the secondary winding. When the primary current is suddenly interrupted, the magnetic field rapidly collapses causing a high voltage pulse across the secondary terminals (an example of electromagnetic induction). Because of the many thousands of turns in the secondary coil this voltage can be several thousand volts, often enough to cause an electric spark across the air gap between the secondary output terminals (originally this type of coil was called a spark coil, and is now called an ignition coil in car ignition systems). The DC supply current must be repeatedly broken, and a magnetically activated vibrating arm (called a break) was mounted on the end of the coil next to the iron core. The magnetic field generated in the core attracts the breaker, opening a contact in the primary circuit. When the magnetic field collapses, the arm springs back to close the primary circuit, turning on again the magnetic field. This is repeated many time per second. Below we can see an early 20th C French Ruhmkorff coil, alongside a close-up of the breaker. To increase the speed of switching, enabling the coil to produce higher voltages, there was usually a paper and metal foil capacitor fitted into the wooded base.

Rühmkorff, as an instrument maker, was also known to have made some of the earliest gas discharge tubes using the so-called “electric egg” electrodes.     

Johann Heinrich Wilhelm Geissler (DE, 1814-1879) was the inventor of the Geissler tube, a glass, low pressure, gas-discharge tube. Around 1858 he started making the tubes for Julius Plücker (DE, 1801-1868), and Geissler quickly became famous for the superior vacuum he was able to obtain using a hand-crank, all-glass, mercury air pump. He was also the first person to use aluminium plates attached to the platinum electrodes, thus avoiding the vapourisation of the platinum and the blackening of the inside of the tubes. His tubes were entertainment items in the 1880’s, and evolved into the neon lighting tube around 1910. It is said that these tubes were the basis for the design of Crookes tubes (1869-1875). Plücker published several papers around 1857 on the action of magnets on electrical discharge in rarefied gases, and he saw the “lichtstrom”, a undulating flickering light at the negative electrode that could be changed into a brilliantly illuminating fine layer (later known as the “negative glow”). He went on to publish a detailed review of his experiments with the Royal Society of London in 1860 (Plücker had been ignored in Germany but had many contacts in England). In 1864 Plücker published a joint paper with Hittorf, again with the Royal Society (the topic was the spectra of gases). Around 1876 Johann Hittorf (DE, 1824-1914) realised that the glow was in fact some type of ray travelling in straight lines through the tube from the cathode (thus the name cathode rays). In 1897 J.J. Thomson (GB, 1856-1940) showed that cathode rays were in fact a new particle called an electron. It is said that the Geissler was instrumental in opening up a new branch of physics that led directly to the discovery of cathode rays, and later X-rays. Below we have two of the simpler, earlier Geissler tubes. The colour of the light emitted depended on the type of gas in the tube.

And below we have a different type of early Geissler tube, possibly from around 1870. And below that we have another Geissler tube with two uranium glass grapes (check out here, here and here for more fantastic ornamental Geissler designs).

In the 1873 Crookes invented a radiometer, or light mill. It had all started when he was weighing chemical samples in a partially evacuated chamber to reduce the effect of air currents, and some of the samples were disturbed by sunlight. In fact this work was performed in the context of Crookes determination of the atomic weight of thallium, an element he had discovered in 1861. The light mill has a low friction spindle with several vertical lightweight metal vanes, all placed in a glass bulb with a partial vacuum. Exposed to sunlight (even the heat of your hand can be enough) it becomes a heat engine, converting thermal heat to mechanical work. The black side of each vane becomes hotter than the other side by adsorbing radiant energy from the light source. This heats up the remaining air molecules on that side of the vane. There must be just enough air molecules left in order to create an air current to propel the vanes and transfer heat to the outside glass bulb before both sides of vane reach thermal equilibrium. Crookes incorrectly thought that this movement was due to the pressure of light. Others have suggested that gas molecules hitting the warmer side of the vane pick up some heat (energy) and “bounce” off the vane with increased speed, exerting a minute pressure on the vane. This would make a slight net push on the black side. There is some truth in this but the force is only sufficient to move the vanes at a very low speed (thus it is not an explanation of the phenomenon).

It is interesting to note that it was Osborne Reynolds (IE, 1842-1912) and Franz Arthur Friedrich Schuster (GB, 1851-1934) who demonstrated in 1876 that the phenomenon of the movement of the light-mill vanes was solely due to the residual gas. And they used a version of the light-mill from Geissler. Here we have to digress to highlight what is perhaps a major un-written law of physics. Reynolds has seen that when the light source was extinguished, the light-mill rapidly stopped moving. Friction was clearly not sufficient to bring the vanes quickly to rest. Reynolds reasoned that it must be due to the residual air, and he also reasoned “what air can prevent, can [also] cause”. It was James Clerk Maxwell (GB, 1831-1879) who proved that the residual gases slid over the surface of the vanes from colder to hotter sites producing a tangential force sufficient to set the vanes in motion. 

Crookes continued (with Charles A.Gimingham) to build different experiments to look at the way streams of molecules could be ejected from surfaces. Did they exert pressure? Could they be made to provide mechanical work? And they soon found that a heater would act as if “a molecular wind” was blowing from it. The focus shifted from the excited surfaces to the void between the vanes and the surface of the glass vessel, and Crookes introduced the concept of “lines of molecular pressure”. He even hung small mica “flies” in the radiometer flask in order to detect molecular movement, and he was able to see movement in certain directions. 

Crookes first announcement concerning what we would understand as a cathode ray tube was in 1879, and by 1888 there were more than 20 different Crookes tubes being sold, although the one containing a diamond needed to be specially ordered. By the early 1900’s there were many manufacturers offering catalogues of cathode ray and X-ray tubes, one included more than 32 pages of different tubes in their catalogue. The one we see most frequently in the different reviews, etc. is the pear-shaped Crookes tube (also often called a Hittorf tube). 

In the above example the electrodes are not “in line”, the cathode is a flat aluminium plate on the narrow side of the “pear” while the anode is rod in a secondary lateral tubular chamber. The other longer tube was for the vacuum. This is the type of cathode tube used by Wilhelm Conrad Röntgen (DE, 1845-1923) in late 1895 (probably, but not totally certain). The unknown, or “X-rays”, were produced by the large area of the glass wall, on the wider side of the tube. The was no focus of the cathode rays, and the large size of the X-ray source explains the lack of sharpness of the early photographs. Also they required long exposure times.

In a publication of 1891 Crookes summarised his understanding on “radiant matter”, i.e. cathode rays or electron beams. It moves in approximate straight lines, almost normal to the surface of the electrode. Below we have one of the experiments of Crookes. The tubes are exhausted to a pressure of 0.001 mm Hg, and separated by a thin glass diaphragm C pieced with two holes, D and E. A´ is the source focused on the hole D. Crookes explains this by saying that a stream of charged molecules is forced by the negative pole through the upper hole, and turns the vanes. And there is a stream of molecules passing back through the lower hole, moving the lower vanes. This return was seen as necessary to close the circuit between poles A and B.     

Crookes went on to describes quite a number of different experiments with discharge tubes, including a “pear-shaped” bulb, the “Maltese Cross”, the “Railway Track”, as well as the fluorescent and phosphorescent behaviour of oxides and minerals.

Before returning to the development of the Crookes tube, and its use by Röntgen to discover X-rays, we must look a bit more carefully at the glow discharge in the tube. So Crookes took a simple two electrode tube, held at low pressure, and created a DC glow discharge. The usual explanation is that a small number of atoms are ionised through thermal collisions in the gas (or by natural radiation background), giving the gas a small conductivity. Ions (positively charged) are driven towards the cathode by the electric potential, just as the electrons are driven towards the anode. Electrons hitting the anode can knock loose secondary electrons, but they will rapidly fall back to the anode. However, the positive ions hitting the cathode can release secondary electrons. It is not efficient, but the electrons produced are in the right place and can support a discharge. These particles will also collide with atoms in the gas, ionising them. Ionisation simply relies on the kinetic energy of the electrons, and it needs the electric field to acquire this kinetic energy. Even a small electric field can be enough, depending upon the electron mean free path and the speed it can acquire before being involved in a collision (all dependent upon the gas pressure). Simply put, the more intense the electric field, the more the electron acquires kinetic energy, the greater the chance it has of causing an ionisation. In neon it will take 41.07 eV to knock out two electrons. Why two electrons? Well for one thing most collisions do not result in ionisation, so the average energy per ion pair produced is usually greater that the ionisation potential, close to twice that value. Atoms and molecules have excited states (resonance energy levels) that are less than the ionisation potential, but these levels are metastable and can “store” energy for extended periods of time, allowing additional collisions to occur and cumulative ionisation (atoms ionised by multiple collisions). A discharge involves the emission of light, so it is all about a competition between the way excitation energy can be lost (de-excitation) before it can be radiated, verses radiation. As more free electrons are created, they can ionise, creating additional electrons. This created an electron avalanche, with a burst of electrons travelling towards the anode. The positive ions are slower as they make their way to the cathode. This multiplies the original electron current, but does not start a sustained discharge. This increase in current produces little light, and is called a dark or Townsend discharge. The ions finally arrive at the cathode, and about 1 in 45 will eject an electron from the cathode surface. However if the electron avalanche produced more than 45 electrons, then there will be an abundance of positive ions arriving at the cathode to replace the electron that originally left the cathode. Now the discharge is producing its own electrons, and has become self-sustaining. At this point the discharge becomes visible, and the potential across the anode and cathode is called the spark potential. We can imagine this as a path between the anode and cathode which becomes conductive because of the electrons and ions distributed along it. A discharge between metal electrodes in a glass tube that gets its electrons from positive-ion bombardment of the cathode, and is confined by the glass walls, is called a glow discharge. Ion bombardment of the cathode will heat it up, and it will begin to emit more electrons. This final state of the discharge is the arc. (Look here for a more complete discussion of electrical discharges).  

Above we have a glow discharge (glow just means emitting light). The Positive Column, if extended, can become striated, with alternating dark and bright bands. The Aston Dark Space is a region which rapidly accelerates electrons away from the cathode, but where electrons also outnumber positive ions, however the electron density and energy is still too low to efficiently excite the gas (thus it is dark). In the Cathode Glow the electrons are now energetic enough to excite neutral atoms. The Cathode (Crookes, Hittorf) Dark Space is a region with a strong electric field, and a high ion density (positive space charge). Here the electrons are creating ions through collisions with gas atoms (not exciting them to emit light). The Negative Glow is the brightest part of the discharge, and where the maximum number of exciting and ionising collisions occur. The Faraday Space, or cathode dark space, is simply when the vacuum is such that the electrons can travel that length before striking a gas atom. It is also where the electron energy is low because the axial electric field is at its lowest. The Positive Column is a luminous region with a small electric field but enough to maintain ionisation near the anode (thus excitation and light emission). The Anode Glow is slightly brighter, and the Anode Dark Space has a high negative space charge due to all the electrons arriving at the anode.    

Now below we have the current-voltage curve (characteristic curve) of a discharge in neon gas at 1 torr. Below the ionisation voltage or breakdown potential there is no glow (A), just random pulses from cosmic radiation. As the voltage increases (A to B) ionisation occurs, a Townsend discharge (B) happens and at one point (D) the discharge becomes self-sustaining and a glow discharge becomes visible. The glow discharge can have a number of differing characteristics. First the discharge will be unstable, and we will see so-called corona discharge (E). This is due to a localised conductive region that often forms around pointed or sharp metal conductors. This leads into a sub-normal glow discharge (F), then a normal glow discharge (G), and then an abnormal glow discharge (H). At one point the cathode glow will start to cover the entire cathode and an arc discharge will begin (J). Initially unstable there is a glow-arc transition (flickering), before a complete electric arc, i.e. complete electrical breakdown and a sustained plasma discharge as the current passes through the normally nonconductive gas (K). You can see the so-called “dark-region”, followed by the glow discharge where most of the light is emitted by excited neutral atoms in the gas, and then there is the arc discharge which produced the most light (we are now in the domain of plasma physics).  

Other types of Crookes tubes included the famous Maltese Cross (below left) and the mineral tubes (below centre and right). The Maltese Cross tube was built by Julius Plücker (DE, 1801-1868), and it was designed to demonstrate that the rays travelled in straight lines by creating a sharp cross-shaped shadow on the fluorescence on the back face of the tube. It also showed at the same time that cathode rays were stopped by metal. Another very popular type of tube demonstrated the fluorescent or phosphorescent behaviour of minerals, e.g. red with calcite, yellow with apatite, bright green with willemite, blue with scheelite, and violet with magnesite.

The one below is perhaps the most interesting, it is the famous Crookes “Railway Tube”. When a high voltage is applied the paddle wheel travels down the glass “railway tracks”. 

Crookes summarised his understanding in 1879 by saying “In studying this fourth state of matter we seem at length to have within our grasp and obedient to our control the little invisible particles which with good warrant are supposed to constitute the physical basis of the universe”.

In 1886 Eugen Goldstein (DE, 1850-1930) discovered that whilst cathode rays flowed in one direction, there was another type of faintly luminous ray travelling in the other direction, he called them “canal rays” or “anode rays”. Today we know that they are positive ions created by the electric field. The name came from the fact that Goldstein had holes in the cathode, and rays appeared to channel through these holes. Initially Goldstein could not detect any deviation in a magnetic field, but in 1907 it was shown that the deflection depended upon the mass of these canal rays. The lightest ones, were about 1840 times the mass of the electron, and were formed when there was some hydrogen gas in the tube. They were protons. Below we have a Goldstein canal ray tube, and we can see the electrons (green) in the lower part of the tube and the red canal rays (protons) in the upper part of the tube. Wilhelm Wien (DE, 1864-1928, won the 1911 Nobel Prize for his displacement law and his work on heat radiation), also made his own Goldstein tubes with additional electrodes so that he could study in detail the positive and negatively charge particles produced.   


Philipp Lenard (DE, 1862-1947, received the 1905 Nobel Prize for his work with cathode rays) began studying gas discharges in 1888. Originally the gas discharge tubes were made of partially evacuated sealed glass tubes. Lenard developed a way to insert small metallic windows in the glass which were thin enough to allow the passage of the rays. The rays could then be studied in the laboratory or in a second vacuum chamber. To detect the rays Lenard used paper sheets coated with phosphorescent materials, similar to the paper and screens used by Röntgen when he discovered X-rays. Below we have a Lenard tube, which was often painted black so as to see better the weak purple cathode rays at the front of the tube (e.g. where there was a thin aluminium foil window).

So prior to Röntgen’s discovery of X-rays there were many types of gas discharge tubes in use, e.g. Geissler, Crookes, Hittorf, and Lenard. After his discovery there was a renewed interest in tubes designed for taking X-ray pictures, best characterised by the so-called “focus tube”. The key feature was that the cathode rays were focussed on a heavy metal (platinum) target rather than on the glass end of the tube itself. With a voltage in excess of 5 kV the electrons (cathode rays) are accelerated enough to create X-rays when they hit the anode or the glass wall of the tube. Many early Crookes tubes certainly produced X-rays, and there were reports of people finding foggy marks on unexposed photographic plates, etc. In the below example (dating from 1896) we can see the concave shape of the cathode providing the focus on to a small area on the angled square platinum target anode. A number of people could claim to have designed this type of tube, but Herbert Jackson (GB, 1863-1936) is often given credit. 

There were a multitude of designs, the one just above is called a Villard tube (ca. 1900), named after Paul Ulrich Villard (FR, 1860-1934), and had three electrodes, an anode and a cathode, and a so-called anticathode (bi-anode tubes). This anticathode was an auxiliary anode, usually given a positive charge (or no charge at all). The anticathode was the target but there were few clear reasons why they were used (possibly to prolong the life of the tube), and they disappeared after about 1920. In any case about this time the hot cathode high vacuum Coolidge tube made the cold cathode tubes obsolete. These were named after William David Coolidge (US, 1873-1975).

Below we have a number of other designs of cold-cathode tubes, many used for laboratory demonstrations. The first has three short spherically tipped anodes, presumably designed to show how the electron beam could be split into three beams. The second one has four electrodes, and was sold in 1908 as a “universal experimental laboratory tube, a combination of a cathode ray tube and an X-ray tube”.

The cold-cathode tubes were “temperamental” and Röntgen is said to have stated “I do not want to get involved in anything that has to do with the properties of tubes, for these things are even more capricious and unpredictable than women”.

The hot-cathode or Coolidge tube used a tungsten thermionic filament as the cathode. Otherwise the basic design was the same. The filament provided the stream of electrons, and there was a high voltage potential placed between the cathode and a water-cooled anode, and there was either an end-window or side-window thin enough to allow the X-ray photons to pass through them. The example below is a “Okco” hot-cathode tube where we can see the focus unit on the cathode filament and the copper heat radiator with the tungsten target set at 45°.

This type of tube would be rated at 6 kV, but after 1918 they were replaced by the so-called “line tube”, or “line filament”. The one below included a “line filament” with a cylindrical focusing reflector, a target set at only 12°-15°, producing a treatment spot of about 3 mm square. This type of tube could be operated up to 80 kV with a tube current of 100 mA. This particular tube had a water-cooled anode, but would have been rated at only about 6-10 kV.

I do not intended to use this article to describe the complete history of the many, many different designs of X-rays tubes. However it is worth noting that an alternative use for the cold-cathode tube was also found, as a rectifier, i.e. a device that converts alternating current (AC) to direct current (DC). They are what has been called thermionic diodes. The very early ones looked just like a cathode ray tube. The heated filament releases electrons into a vacuum and the alternating voltage (to be rectified) is applied between the cathode and some form of plate anode. When the anode has a positive voltage the electrons from the cathode are collected, and the current flows. When the polarity on the anode is reversed, the electrons are repelled, and no current flows. In the very early days (below we have a rectifier made by Oliver Joseph Lodge (GB, 1851-1940) in the early years of the 1900’s) the rectifier was a cold-cathode tube. A long, wide aluminium spiral was one electrode (cathode), and the other was a small metal rod (the anode). Here the rectification occurred simply because of the important size asymmetry of the electrodes, with a current flowing much more easily in one direction than in the other. When the large electrode is negative a large flow of electrons converges on the small positive electrode, but when the small electrode is negative, the electron emission is low, and the flow to the large positive spiral electrode is negligible.       


A final, and worthwhile point, is that Karl Ferdinand Braun (DE, 1850-1918), who shared the 1909 Nobel Prize with Marconi, developed the first cold-cathode ray tube with magnetic beam deflection. It was based upon the deflection tube of Crookes, and was built by the successor to Geissler. Braun was able to use it to visualise alternative current already in 1897, and thus it was the first oscilloscope. Below we can see a Braun tube from the early 1900’s, it had an internal mica screen covered with phosphorescent paint. The long neck had a very small hole for the electron beam, and a electromagnet was used to produce a spot on the screen.

I have read that the first idea to use Braun’s tube as a kind of television was from Alan Archibald Campbell-Swinton (GB, 1863-1930) in 1908. I have also seen references to the first transmission of images by Georges Rignoux and A. Fournier, and I have seen mention of Boris Lvovich Rosing (RU, 1869-1933) and his “electric telescope” as early as 1897. My understanding is that they were all using a version of the Braun tube. Edwin Belin and Fernand Holweck (FR, 1890-1941) were said to have used a cathode ray tube as a television receiver and demonstrated it in 1928.

What better way to close this section on early cathode ray and X-ray tubes than with the tube used by J.J. Thomson in 1897 to prove that the stream of particles carried a negative charge and were about 2000 times lighter that the hydrogen atom. These were electrons. This tube was developed just before that of Braun, and it had two internal electrostatic deflection plates, whereas Braun used magnetic deflection.

And to conclude this short summary of the work of Crookes and the development of the gas-discharge tube, it is also worth noting that he was quite a prolific scientist. Crookes discovered thallium, which he made with the help of spectroscopy (an unknown element with a bright green spectral line). He also identified the first known sample of helium in 1895. In 1903 he separated from uranium a new element which was later identified and called protactinium. He also observed that particles ejected from radioactive substances, impinging upon zinc sulfide produced a minute scintillation. This device was called a spinthariscope, and we know now that he observed individual α-particles colliding with zinc sulfide and each collision producing scintillation of light. Today the scintillation counter is one of the most important instruments for detecting and measuring ionising radiation. If you want to know more about Crookes impressive array of achievements have a look at “William Crookes (1832-1919) and the Commercialization of Science” by William H. Brock (2008). 

Some very useful reference material:

Introduction to Gas Discharge Tubes and Cold Cathode X-Ray Tubes”, “X-Ray and Gas discharge Tubes” are all part of the Oak Ridge Associated Universities Health Physics Historical Instrumentation Museum Collection.

Antique X-Ray Tubes and Accessories” from Dr. Hakim’s collection

The Cathode Ray Tube Site” by Henk Dijkstra


Johann Wilhelm Hittorf (D, 1824-1914) is perhaps best known today for his work on electrical discharges in a vacuum tube (he was mentioned in the Nobel Lecture of Röntegen).

But first I will rapidly look at his work on the migration of ions during electrolysis. Along with Michael Faraday (GB, 1791-1867) and Friedrich Wilhelm Georg Kohlrausch (DE, 1840-1910), he was considered (at that time) one of the fathers of electro-chemistry, a branch of physical chemistry. This is all about the chemical reactions that occur at the face of a metal electrode in a (non-metallic) ionic conductor, or electrolyte. I said “at that time” because the excellent Wikipedia review on electro-chemistry only mentions Faraday with his two laws of electrochemistry (or electrolysis), and ignores both Hittorf and Kohlrausch. The reason that Hittorf was mentioned “at that time” was because of his body of experimental work and the explanation he gave for the concentration changes produced at the electrodes during electrolysis. Kolhrausch was mentioned because he recognised the value of the work of Hittorf in his formulation of the law of independent migration of ions. The work of these three lead to the theory of solutions formulated by Jacobus Henricus van’t Hoff (NL, 1852-1911), and the dissociation theory of Svante August Arrhenius (SE, 1859-1927).

Hittorf appears to have built on the work of Theodor von Grotthuss (DE, 1785-1822), who was the first to explain the process of electrolysis. But it was Faraday who discovered the fundamental electrolytic action of the current, e.g. that electricity passing through an electrolytic cell was carried by the movement of charged ions produced from the decomposition of the compounds making up the solution. Hittorf also mentioned the work of John Frederic Daniell (GB, 1790-1845) and William Hallowes Miller (GB, 1801-1880) who concluded that those metals which decomposed water at ordinary temperature, or whose oxides were easily soluble in water, were subject to a progressive transference in the voltaic cell from the anode to cathode during electrolysis (e.g. salts such as potassium sulphate, barium nitrate, magnesium sulphate), and those which did not possess a strong affinity for oxygen retained their place (e.g. copper and zinc). Claude Servais Mathias Pouillet (FR, 1790-1868) performed an experiment with a gold solution in a U-shaped tube. 

He found the solution in the negative arm (anions are positively charged and are attracted to the negative arm) was almost completely deprived of its gold, while that in the positive arm (cations are negatively charged and are attracted to the positive pole) still contained the original gold content. He and many others (incorrectly) thought that this was explained by the fact that all the chemical forces (decomposing actions) resided at the negative pole. They thought that the negative pole must have decomposed the gold solution, absorbing the gold (removing it from the solution) and sending the chlorine through a series of decompositions and recombinations to the positive pole, to be set free).

Hittorf correctly pointed out that a dilution at the negative pole did not prove that the gold has not migrated during electrolysis. Cations migrate towards the cathode and away from the anode, and the deposition of the anion of the positive electrode, together result in a decrease of the salt in the neighbourhood of the anode. The same process also produces a decrease in the concentration of the salt in the neighbourhood of the cathode. If the motion of the two dissimilar ions were the same, the decrease in the concentration of the salt would be the same at the two electrodes. He built an experiment that showed that the concentrations at the two electrodes were not the same, therefore the speed of migration of cations and anions were different. This clearly meant that different ions would contribute differently to an electric current, e.g. differences in the transport number arise from differences in electrical mobility. He went on to publish laws covering the migration of ions.  

We have to remember here that the anode is the place where the current enters the electrolyte, and where electrons leave the cell. Therefore outside the cell, electrons flow from the anode to the cathode, and inside the electrolyte electrons flow from the cathode to the anode.

This process can be seen in gold electro-plating. The gold from the gold anode is oxidized and dissolves in solution as Au3+. The electrons arriving at (say) an aluminum glass frame cathode reduce the Au3+ in solution to Au (solid) on the surface of the frame cathode (it is connected to the negative terminal and is thus a source of electrons). This same process can be used to recover gold from things such as motherboards, plated jewellery, etc. The process is called reverse electro-plating. In this case you need to use an acid to dissolve the gold so it can be collected. You simple connect the positive terminal (representing the anode) of (say) a battery charger (thus direct current) to the gold items. People often use a copper dish to put all the gold items on. The cathode (connected to the negative terminal of the battery charger) is usually stainless steal, from which the gold plating can be easily pealed off.   

Luigi Valentino Brugnatelli (IT, 1761-1818) is attributed with inventing electro-deposition in 1805, using the voltaic pile, invented by Alessandro Giuseppe Antonio Anastasio Volta (IT, 1745-1827) in 1799. However it is also said that Johann Wilhelm Ritter (DE, 1776-1810) also discovered electro-plating in 1802, and built a electro-chemical cell with 50 copper discs separated by cardboard disks moistened with a salt solution.

Why spend so much time and effort on electrolysis? Well it is a technique that uses a direct electric current to drive otherwise non-spontaneous chemical reactions, which, for example, is important in separating chemical elements from ores. You dissolve something, pass a DC current through it, produce chemical reactions at the electrodes, and collect the atoms. There are numerous industrial applications, e.g. electro-metallurgy, electro-platting, electrolytic etching, refining of copper, and the production of oxygen for spacecraft and nuclear submarines. A modern-day topic related to electrolysis is the electrochemical cell, e.g. the hydrogen fuel cell.  Another modern-day application is the ion exchange membrane used for desalination, wastewater treatment, and chemical recovery applications.

Looking back 100 years, classical theories of solutions were built upon the analogy between the solute particles (i.e. solids or particles dissolved in a liquid) and the molecules of an imperfect gas, the solvent being regarded as a mere provider of the volume in which solute particles moved. Today the modern theory of liquids is now based upon a disordered solid in which short-range order persists, but long-range order, characteristic of a solid, has been lost in thermal agitation. As such solute and solvent now appear of equal footing. However ions moving in a solution, under normal conditions, do so as a function of the applied field (there is no simple relationship between velocity and mass). The ionic conductivity, often referred to as ionic mobility, is simply related to the mobility of the ions. The passage of electric current through an electrolyte solution is just the motion of ions of opposite charge moving in opposite directions under an applied potential. And the transport number tells us the fraction of the total current carried in an electrolyte by a given ion, and a difference in transport number arises from a difference in electrical mobility. The method used by Hittorf to determine the transport number is comparable with a fuel cell under operation. The two compartments in the cell are separated by an ion exchange membrane and both compartments are filled with the same solution, with the same concentration. A current is applied, and cations and anions present in the solution migrate through the membrane in order to maintain electro-neutrality. In the case of a fuel cell, hydrogen and oxygen constantly flow into the cell, migrate through the membrane, are converted into water, and in the process produces electricity.  

Let us return to the work of Hittorf on electrical discharges in a vacuum tube. In 1838 Michael Faraday (GB, 1791-1867) had studied the conduction of electricity through solutions and gases. But progress could only be made when Johann Heinrich Wilheim Geissler (DE, 1814-1879) developed in 1855 the hand-cranked mercury pump. In 1858 Julius Plücker (DE, 1801-1868), who was a friend of Geissler, used these new Geissler tubes and found that cathode glow would follow the “lines of force” of magnetic fields, and that the glass walls of the tube fluoresced near the cathode (the fluorescence could also be moved with a magnetic field). Many scientists of the period studied cathodes rays (electrons) in gases, including William Crookes, Johann Hittorf, Julius Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard (Nobel Prize winner) and others. The discovery of the properties of cathode rays, culminated in J.J. Thomson (GB, 1856-1940) identifying in 1897 cathode rays as negatively charged particles, which were later named electrons (for which he also won the Nobel Prize). The term itself, cathode rays (“Kathodenlicht” and “Kathodenstrahlen”), was used first by Eugen Goldstein (DE, 1850-1930) in 1876 and again in 1880.

Following on from the work of Plücker, Hittorf (who was Plücker’s student) determined that any solid or fluid, whether conductor or insulator, cut off the glow at the cathode. He determined that the glow was from a point cathode and travelled in straight lines (“strahlen” or “rays of glow” with the cathode as the point of the cone). I found one reference that mentioned that Hittorf used a Sprengel pump, after Hermann Sprengel (DE/GB, 1834-1906), and was able to obtain a pressure less than 0.001 mbar. Hittorf also found a “negative glow” and a “positive glow” of different colours, that propagated in opposite directions. Over about a 20 year period Hittorf (like Crookes) made a multitude of different shaped vacuum tubes, varied the location of the electrodes and the gases and pressures used, and amassed a wealth of information about cathode rays. Some people have suggested that it was Hittorf that, based upon his vast expertise, first suggested that light seen in a discharge tube came from a wave-like phenomena in the aether, thus starting a 30 year controversy that finished by the negation of the wave theory in favour of the particulate view (see wave-particle duality).  

It is clear that Hittorf and Crookes exchanged much information on their work. In fact Wilhelm Conrad Röntgen (DE, 1845-1923) used a so-called Hittorf-Crookes tube for his discovery of X-rays. Röntgen was using several different types of Hittorf-Crookes tubes, as well as Lenard tubes, and the one he (probably) used to discover X-rays was the so-called “pear-shaped tube”. Tube designs for obtaining X-rays photographs (around 1896) changed so rapid that a particular design could go out of date within 6 months. The Hittorf-Crookes tube was by no means optimal, but the images obtained by Röntgen were exceptional. In the tube the size of the focal point on the glass wall was rather diffuse, and lead diaphragms had to be used to obtain clear pictures (but with long exposure times). Some of the first tubes also heated the glass wall, making it impossible to increase the load on the tube and reduce the exposure time. But scientist of the period were ingenious in finding new ways to overcome these problems. For more information on the early evolution of these tubes, checkout my entry for Röntgen. 


Eugen Goldstein (D, 1850-1930)

We often forget that whilst an effect may be discovered by one person, many others confirm and elaborate that early discovery. Goldstein built his own Geissler tube and Crookes tubes and confirmed the rectilinear propagation of the rays and the influence of a magnet.

In fact Goldstein was the first to use the term cathode rays (“Kathodenlicht” and “Kathodenstrahlen”). He then bored holes in the cathode and was surprised to see a golden-yellow light appear on the wall of the glass vessel situated on the other side of the cathode. He found that it was due to rays of a new kind propagating in the direction opposite to that of the cathode rays, and he called them canal rays (“Canalstrahlen”). We must remember that cathode rays (electrons) move from the negatively charged cathode towards the positively charge anode, but these canal rays moved in the opposite direction.

Below we can see the negative electrode plate with holes in. What we have are positively charged atoms accelerated in an electric field toward the electrode plate. The so-called canal rays” pass through the holes in to the "field-free" space. The pink light is because of collisions with the helium low-pressure gas filling.

Some experts thought these rays were aether waves. Heinrich Rudolf Hertz (DE, 1857-1894) saw that the colour depended upon the operating conditions of the tube, and he thought that each colour corresponded to a different type of cathode ray. It turned out that these canal rays are positive ions whose identity depended upon the nature of the residual gas in the tube. Wilhelm Carl Werner Otto Fritz Franz Wien (DE, 1864-1928) studied these canal rays, and his work would later become part of the basis for mass spectrometry. For example, H+ ions from the anode (this ion has the smallest e/m ratio) are actually nothing more than protons. And thus Goldstein’s observation of this ion species might possibly have been the first observation of the proton.

Goldstein also went on to discover that cathode rays were emitted perpendicularly from a metal surface, and that they carried energy.