History: The cathode rays

THE CATHODE RAYS

The fact that electrostatic generator caused sparking takes a longer distance in the rarefied air then in standard air was noticed and described at first in 1705. Over a century later, in 1838, Michael Faraday passed current through the rarefied air filled glass tube. Conducting the experiment he noticed a strange light arc with its beginning at anode (the positive electrode) and its end almost at cathode (the negative electrode). The only place where there was no luminescence was just in front the cathode. It is called "cathode dark space", "Faraday dark space" or "Crookes dark space".

That was the beginning of the long and "turbulent" time of that luminescence researching. And the luminescence is called "cathode rays" (named by Eugen Goldstein).

Geissler in 1855, discovered a new kind of vacuum pump, in which the column of mercury was the piston. Thanks to the invention lower and lower pressure was achieved what, in turn, let the researches on cathode rays develop (the rays were surveyed in the vacuum tube).

Juliusz Plucker (1801-1868),in 1858, remarked that as the pressure of the gas in the tube decreases, the length of the luminescence by the cathode increases. The direction of the lines of cathode rays is the same as the magnetic field lines' one. He also noticed that if the cathode was made of platinum then the part of the glass tube near the cathode got covered with that metal. Whereas the glass in front the cathode began to illuminate. The location and the shape of that illumination was due to the magnetic field.

The discharge tube used by Plucker for creating the cathode rays

Johann Wilhelm Hittorf (1824-1914) in 1869, ascertained that the cathode rays propagated in straight lines (when there was no magnetic field of course) He noticed that when there was any object placed between the cathode and the illuminating side of the tube, then the shadow of that object appeared.

Eugen Goldstein (1850-1931) in the seventies of the 19th century he was examining the features of cathode rays. He noticed that the cathode rays were emitted perpendicular with respect to the surface of the cathode on the contrary to the shafts which propagate in all directions. He ascertained that the concave cathode (of the bowl shape) emitted the cathode rays which all focused in one point. He also proved that the features of that rays were not due to the material of which cathode was made. Moreover the cathode rays can induce the chemical reactions which are normally caused by the sun light (photochemical reactions).

Cromwell Fleetwood Varley (1828-1883), in 1871, came to the opinion that cathode rays could consist of the negatively charged particles. That particles would be deflected by the magnetic field in the same direction as the observed direction of the cathode rays deflection.

Sir Williama Crooks (1832-1919), built up Varley'a conception. Crooks conducted many important experiments using self-made vacuum tubes. He noticed that the thin foil on which the beam of cathode rays was focused got hot. That proved that the rays, whatever they were, transferred energy. The second thing he discovered was that the beam of rays exerted some force - transferred momentum. He demonstrated that using the paddle wheel which he put inside the vacuum tube. The paddles were in such direction as to be influenced by the rays' incidence. The wheel could roll in the tube when there was some force influencing the paddles (the friction was minimized). The tube laid horizontal. The wheel began to move when the cathode rays illuminated the paddles. For Crooks that movement proved that the cathode rays influenced the paddles with some force. But, in 1903, in his book "Conduction of Electricity Through Gases" Thomson proved that the force with which the cathode rays influenced the paddles was not strong enough to induce such a fast movement. So Thomson proved that the movement was really induced by the radiometric effect - the paddles were not uniformly heated - The heated and unheated sides of the paddles received different momentum from the particles of the gas in the tube. The only thing proven by the Crooks's experiment was that cathode rays heated the paddles. But in the eighties the experiment was treated the proof for cathode rays transferring momentum.
Crooks was studying the structure of the cathode rays and the reason for them being induced. In the model, he created, the particles of the tail gas in the tube collided with the cathode and that way got negatively charged. After that the particles were repulsed by the cathode, getting high speed. That was because the cathode and the particles were the same charged. The particles, repulsed perpendicular with the respect to the cathode, passed through the cathode dark space and then induced illumination by collisions with the other particles. Such model explained the most of the cathode rays' features and phenomena.

Tait yet in 1880 noticed a big mistake in the Crooks's theory although the theory developed in the fall of 70ths of the 19th century. Tait noticed that if cathode rays were really fast moving particles then the light waves emitted by them should be characterized by Doppler shift. They weren't.

Wiedemann and the two other German scientists - Eugen Goldstein and Heinrich Hertz - created a different model explaining features of cathode rays. They came to the opinion that cathode rays couldn't consist of particles but were of wave structure. Their conclusion was caused by the fact that all features of cathode rays were the same as of electromagnetic waves. The difference laid only in the two things: the first is that waves don't undergo aberration in the magnetic field and as cathode rays do, and the second is that the waves are emitted in all directions with respect to the surface and the cathode rays are emitted only perpendicular with respect to the surface of the cathode. The authors of the theory stated that the differences could be explained by some unknown features of eteru and by the electrical nature of the rays creation.

    And so in the second half of the 19th century there were two models describing cathode rays. They both explained some phenomena and both had trouble with some other ones. The scientists differed their opinions and broke into the two groups - the first one believing in the corpuscular model and the second one in the wave model being the right one. There were many interesting experiments conducted to prove which of the hypothesizes was right. The experiments made some more features of the cathode rays known.
    OOne of the authors of the wave model - Eugen Goldstein - conducted some interesting experiments to prove his theory. He found out that at the given residual pressure in the vacuum tube the distances between the collisions of the electrified particles (the Crooks' particles) colliding the gas particles should be (according to the theoretical calculations) multiplied shorter than the observed "Crookes dark space". Then as Crookes said - the dark space was created where the collisions don't proceed. What Goldstein also remarked is that the distance traveled by cathode rays from the cathode to the end of the vacuum tube was more than 150 times longer than the gas particles' mean free path calculated theoretically. The probability that any Compton's particle would make that distance without collision is like 1 to 10⁶⁵ ! By Goldstein only waves could make that distance not getting scattered, and creating a fluorescing spot on the end of the tube.
    Another important experiment conducted by Goldstein proved that there is really no Doppler shift of the light caused by cathode rays. He constructed a L - shaped vacuum tube. In that tube both the A and the B electrode could act as a cathode. When the A one is the cathode the spectroscope should record the light of the particles getting close (the Doppler shift should occur) When the B one is the cathode the spectroscope recorded light is caused bby the particles moving perpendicular with the respect to the spectroscope (there should be no Doppler shift). Whereas Goldstein changing the function of the cathode between the A and B electrode noticed no change of the spectrum. If cathode rays really consist of the particles being the source of the light then according to the results of the experiments they should move not faster than 23 km/s.

Artur Schuster, who was English was one of the most important sympathizer of the corpuscular theory for the cathode rays. In his opinion not the moving particles were the source of light but the immovable particles of the gas with which the particles of rays collide. That is why, as he said, the Doppler shift doesn't occur. Also the created by Schuster model of the particles was different; the atoms of the gas dissociate for the positive and negative parts. The positive particles are collected by the cathode and the negative parts are repulsed from it - they create the beam of the cathode rays. He conducted also the experiment where he estimated the maximum and minimum limits of the q/m. (where q is the charge, m. is the mass of the hypothetical particle.

Heinrich Hertz (1857-1894) tried to refute the corpuscular model of the cathode rays. The aim of his first experiment was to prove that the cathode rays can be created perpetually (constructing a special power supply he created constant voltage between the electrodes). With such supply the pulsation (noticed before by the other scientists) did not occcure - the rays were emitted perpetually (under the limit of the measuring error). He was of the opinion that the result can be the point against the corpuscular theory for the rays.

The experimental system made by Hertz for detecting the electrical charge

Another aim of his experiments was to prove that the trajectory of the cathode rays movement doesn't have to overlap with the direction of the flow of current. He constructed a machine in which the electrodes were perpendicular . The beam of the cathode rays goes perpendicular with the respect to the cathode and the current flows from the cathode to the anode (as the picture shows). The value and the direction of the flow of current he calculated using a small magnetic needle hung down inside the machine. After conducting the experiments he came to the opinion that it is true that the trajectory of the cathode rays movement doesn't have to overlap with the direction of the flow of current.
In the next of his experiments he was trying to prove that cathode rays don't transfer any charge. So he made a new machine. It is shown on the picture below. The system consists of a vacuum tube with the cathode and anode inside. The cathode rays emitted at the cathode are passing through a small slot in the anode and through a wire net (the net has connections with the anode, and it's task was to screen the rest of the tube from electrode influence - electric field was only between the anode and the cathode). The cathode rays going out from the space between the electrodes were flowing through the rest of the tube and incidenced its end. The tube was put inside the sensitive electrometer to detect the charge. If the cathode rays in the tube were transferring any charge the electrometer should detected that. Whereas Hertz noticed only some small, irregular swings of the pointer. His conclusion was that these were secondary effects and that there was no charge of which the particles could be characterized detected.

One more experiment with the aim to prove that the cathode rays don't transfer the charge . He tried to notice whether the rays were undergoing aberration in the transverse electric field. If the rays would be charged the would undergo the aberration. Hertz put two metal strips inside the tube. Those strips were connect to the battery (between battery and the strips there was resistor of a big resistance). The electric field intensity between strips was small. Hertz didn't observe any curving of cathode rays.
Using the data from the experiment and the value of the aberration measured in the magnetic field he estimated the velocity of the charged particles of which the cathode rays could consist would have to be equal over 1,1 *10⁸ meters per second.
In 1891 Hertz noticed that the cathode rays can pass through a thin layer of metal. He covered a glass shield which contained uranium with a thin gold foil. The cathode rays activated the uranium consisting glass for illumination. When the rays incidenced the layer of gold the glass began to undergo fluorescence . He noticed that the phenomena didn't occur when there was also a thin layer of mica covering the gold. The cathode rays can not only go through gold but also as Hertz showed it through silver, aluminum and alloys of gold or silver and tin, zinc or copper.

Phillip Lenard (1862-1947) projected cathode rays out of a vacuum tube. He used the phenomena discovered by his teacher - the cathode rays an go through the aluminum foil (the cathode rays can go outside but the particles of air can't go inside). Lenard noticed that in the air the cathode rays could make a distance of about 1 centimeter. The gold foil penetration proves that the cathode rays consist of some kind of particles, that are much smaller than atoms.

Since 1705 scientists have discovered many features of the cathode rays. Such great scientists like . Goldstein, Schuster, Hertz and Lenard were studying them. They were for the two different, competitive theories for the cathode rays' behavior - the wave and the corpuscular one. Only just in 1897 the more exact model of cathode rays was formulated. You can read about that in the chapter "The Thomson's experiment".