A
Crookes tube is an early experimental electrical discharge tube, invented by English physicist
William CrookesSir William Crookes, OM, FRS was a British chemist and physicist who attended the Royal College of Chemistry, London, and worked on spectroscopy...
and others around 1869-1875, in which
cathode rayCathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and travelling perpendicular to the cathode Cathode...
s, that is
electronThe electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, were discovered.
A development of the
Geissler tubeA Geissler tube is an early gas discharge tube used to demonstrate the principles of electrical glow discharge. The tube was invented by the German physicist and glassblower Heinrich Geissler in 1857...
, it consists of a partially
evacuatedIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
glass container of various shapes, with two metal electrodes, one at either end. When a
high voltageThe term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
is applied between the electrodes,
cathode rayCathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and travelling perpendicular to the cathode Cathode...
s (
electronThe electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s) travel in straight lines from the
cathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
to the
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
. It was used by Crookes, Johann Hittorf, Juliusz Plücker,
Eugen GoldsteinEugen Goldstein was a German physicist. He was an early investigator of discharge tubes, the discoverer of anode rays, and is sometimes credited with the discovery of the proton.- Life :...
, Heinrich Hertz,
Philipp LenardPhilipp Eduard Anton von Lenard , known in Hungarian as Lénárd Fülöp Eduárd Antal, was a Hungarian - German physicist and the winner of the Nobel Prize for Physics in 1905 for his research on cathode rays and the discovery of many of their properties...
and others to discover the properties of cathode rays, culminating in
J. J. ThomsonSir Joseph John "J. J." Thomson, OM, FRS was a British physicist and Nobel laureate. He is credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer...
's 1897 identification of cathode rays as negatively-charged particles, which were later named
electronThe electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s. Crookes tubes are now used only for demonstrating cathode rays.
Wilhelm Röntgen discovered
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s using the Crookes tube in 1895. The term is also used for the first generation,
cold cathodeA cold cathode is a cathode used within nixie tubes, gas discharge lamps, discharge tubes, and some types of vacuum tube which is not electrically heated by the circuit to which it is connected...
X-ray tubeAn X-ray tube is a vacuum tube that produces X-rays. They are used in X-ray machines. X-rays are part of the electromagnetic spectrum, an ionizing radiation with wavelengths shorter than ultraviolet light...
s, which evolved from the experimental Crookes tubes and were used until about 1920.
How a Crookes tube works
Crookes tubes are
cold cathodeA cold cathode is a cathode used within nixie tubes, gas discharge lamps, discharge tubes, and some types of vacuum tube which is not electrically heated by the circuit to which it is connected...
tubes, meaning the they do not have a heated filament in them that releases
electronThe electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s like the later electronic
vacuum tubeIn electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s usually do. Instead, electrons are generated by the
ionizationIonization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
of the residual air by a high
DCDirect current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
(from a few kilovolts to about 100 kilovolts) applied between the electrodes, usually by an
induction coilAn induction coil or "spark coil" is a type of disruptive discharge coil. It is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current supply...
(a "Ruhmkorff coil"). The Crookes tubes require a small amount of air in them to function, from about 10
−6 to 5×10
−8 atmosphere (7×10
−4 - 4×10
−5 torr or 0.1 - 0.005
pascalThe pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
).
When high
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
is applied to the tube, the
electric fieldIn physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
accelerates the small number of electrically charged
ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s always present in the gas, created by natural processes like radioactivity. These collide with other gas
moleculeA molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s, knocking electrons off them and creating more positive ions in a chain reaction. All the positive ions are attracted to the
cathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
or negative electrode. When they strike it, they knock large numbers of electrons out of the surface of the metal, which in turn are repelled by the cathode and attracted to the
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
or positive electrode. These are the
cathode rayCathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and travelling perpendicular to the cathode Cathode...
s.
Enough of the air has been removed from the tube that most of the electrons can travel the length of the tube without striking a gas molecule. The high voltage accelerates these low-mass particles to a high velocity (about 37,000 miles per second, or 59,000 km/s, about 20 percent of the
speed of lightThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, for a typical tube voltage of 10 kV). When they get to the anode end of the tube, they have so much
momentumIn classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
that, although they are attracted to the anode, many fly past it and strike the end wall of the tube. When they strike atoms in the glass, they knock their
orbital electronsAn electron orbital may refer to:* An atomic orbital, describing the behaviour of an electron in an atom* A molecular orbital, describing the behaviour of an electron in a molecule- See also :...
into a higher
energy levelA quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
. When the electrons fall back to their original energy level, they emit light. This process, called
fluorescenceFluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
, causes the glass to glow, usually yellow-green. The electrons themselves are invisible, but the glow reveals where the beam of electrons strikes the glass. Later on, researchers painted the inside back wall of the tube with a
phosphorA phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
, a fluorescent chemical such as
zinc sulfideZinc sulfide is a inorganic compound with the formula ZnS. ZnS is the main form of zinc in nature, where it mainly occurs as the mineral sphalerite...
, in order to make the glow more visible. After striking the wall, the electrons eventually make their way to the anode, flow through the anode wire, the power supply, and back to the cathode.
The above only describes the motion of the electrons. The full details of the action in a Crookes tube are complicated, because it contains a nonequilibrium
plasmaIn physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
of positively charged
ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s,
electronThe electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, and neutral
atomThe atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s which are constantly interacting. At higher gas pressures, above 10
−6 atm (0.1 Pa), this creates different colored glowing regions in the gas, depending on the pressure in the tube (see diagram). The details were not fully understood until the development of plasma physics in the early 20th century.
History
Crookes tubes evolved from the earlier
Geissler tubeA Geissler tube is an early gas discharge tube used to demonstrate the principles of electrical glow discharge. The tube was invented by the German physicist and glassblower Heinrich Geissler in 1857...
s, experimental tubes which are similar to modern neon tube lights. Geissler tubes had only a low vacuum, around 10
−3 atm (100
PaThe pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
), and the electrons in them could only travel a short distance before hitting a gas molecule. So the current of electrons moved in a slow
diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
process, constantly colliding with gas molecules, never gaining much energy. These tubes didn't create beams of cathode rays, only a pretty glow discharge that filled the tube as the electrons struck the gas molecules and excited them, producing light.
Crookes (among other researchers) was able to evacuate his tubes to a lower pressure, 10
−6 to 5x10
−8 atm, using an improved Sprengel mercury
vacuum pumpA vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...
made by his coworker Charles A. Gimingham. He found that as he pumped more air out of his tubes, a dark area in the glowing gas formed next to the cathode. As the pressure got lower, the dark area, called the
Crookes dark space, spread down the tube, until the inside of the tube was totally dark. However, the glass envelope of the tube began to glow at the anode end.
What was happening was that as more air was pumped out of the tube, there were fewer gas molecules to obstruct the motion of the electrons, so they could travel a longer distance, on average, before they struck one. By the time the inside of the tube became dark, they were able to travel in straight lines from the cathode to the anode, without a collision. They were accelerated to a high velocity by the electric field between the electrodes, both because they didn't lose energy to collisions, and also because Crookes tubes required a higher
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
. By the time they reached the anode end of the tube, they were going so fast that many flew past the anode and hit the glass wall. The electrons themselves were invisible, but when they hit the glass walls of the tube they excited the atoms in the glass, making them give off light or fluoresce, usually yellow-green. Later experimenters painted the back wall of Crookes tubes with fluorescent paint, to make the beams more visible.
This accidental fluorescence allowed researchers to notice that objects in the tube, such as the anode, cast a sharp-edged shadow on the tube wall. Johann Hittorf was first to recognise in 1869 that something must be travelling in straight lines from the cathode to cast the shadow. In 1876,
Eugen GoldsteinEugen Goldstein was a German physicist. He was an early investigator of discharge tubes, the discoverer of anode rays, and is sometimes credited with the discovery of the proton.- Life :...
proved that they came from the cathode, and named them
cathode rays (
Kathodenstrahlen).
At the time, atoms were the smallest particles known, the electron was unknown, and what carried
electric currentElectric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
s was a mystery. Many ingenious types of Crookes tubes were built to determine the properties of cathode rays (see below). The high energy beams of pure electrons in the tubes revealed their properties much better than electrons flowing in wires. The colorful glowing tubes were also popular in public lectures to demonstrate the mysteries of the new science of electricity. Decorative tubes were made with fluorescent minerals, or butterfly figures painted with fluorescent paint, sealed inside. When power was applied, the fluorescent materials lit up with many glowing colors.
In 1895, Wilhelm Röntgen discovered
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s emanating from Crookes tubes. The many uses for X-rays were immediately apparent, the first practical application for Crookes tubes.
Crookes tubes were unreliable and temperamental. Both the energy and the quantity of cathode rays produced depended on the pressure of residual gas in the tube. Over time the gas was absorbed by the walls of the tube, reducing the pressure. This reduced the amount of cathode rays produced and caused the voltage across the tube to increase, creating 'harder' more energetic cathode rays. Soon the pressure got so low the tube stopped working entirely.
The electronic
vacuum tubeIn electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s invented later around 1906 superseded the Crookes tube. These operate at a still lower pressure, around 10
−9 atm (10
−4 Pa), at which there are so few gas molecules that they don't conduct by
ionizationIonization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
. Instead, they use a more reliable and controllable source of electrons, a heated filament or
hot cathodeIn vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. In the accelerator community, these are referred to as thermionic cathodes. The heating element is usually an electrical filament...
which releases electrons by
thermionic emissionThermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the binding potential, also known as work function of the metal. The charge carriers can be electrons or ions, and...
. The ionization method of creating cathode rays used in Crookes tubes is today only used in a few specialized gas discharge tubes such as
krytronThe krytron is a cold-cathode gas filled tube intended for use as a very high-speed switch, and was one of the earliest developments of the EG&G Corporation. It is somewhat similar to thyratron...
s.
The technology of manipulating electron beams pioneered in Crookes tubes was applied practically in the design of vacuum tubes, and particularly in the invention of the
cathode ray tubeThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
by Ferdinand Braun in 1897.
The discovery of X-rays
When the voltage applied to a Crookes tube is high enough, around 5,000
voltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s or greater, it can accelerate the electrons to a fast enough velocity to create X-rays when they hit the anode or the glass wall of the tube. The fast electrons emit X-rays when their path is bent sharply as they pass near the high electric charge of an atom's
nucleusThe nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
, a process called
bremsstrahlungBremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
, or they knock an atom's inner electrons into a higher
energy levelA quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
, and these in turn emit X-rays as they return to their former energy level, a process called
X-ray fluorescenceX-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...
. Many early Crookes tubes undoubtedly generated X-rays, because early researchers such as
Ivan PulyuiIvan Pulyui was a Ukrainian-born physicist, inventor and patriot who has been championed as an early developer of the use of X-rays for medical imaging...
had noticed that they could make foggy marks on nearby unexposed
photographic platePhotographic plates preceded photographic film as a means of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...
s. On November 8, 1895, Wilhelm Röntgen was operating a Crookes tube covered with black cardboard when he noticed that a nearby fluorescent screen glowed faintly. He realized that some unknown invisible rays from the tube were able to pass through the cardboard and make the screen fluoresce. He found that they could pass through books and papers on his desk. Röntgen began to investigate the rays full time, and on December 28, 1895 published the first scientific research paper on X-rays. Röntgen
was awarded the first
Nobel Prize in PhysicsThe Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
(in 1901) for his discoveries.
The medical applications of X-rays created the first practical use for Crookes tubes, and workshops began manufacturing specialized Crookes tubes to generate X-rays, the first X-ray tubes. The anode was made of a heavy metal, usually
platinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
, which generated more X-rays, and was tilted at an angle to the cathode, so the X-rays would radiate through the side of the tube. The cathode had a concave spherical surface which focused the electrons into a small spot around 1 mm in diameter on the anode, in order to approximate a point source of X-rays, which gave the sharpest radiographs. These cold cathode type X-ray tubes were used until about 1920, when they were superseded by the
hot cathodeIn vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. In the accelerator community, these are referred to as thermionic cathodes. The heating element is usually an electrical filament...
Coolidge X-ray tube.
Experiments with Crookes tubes
Crookes tubes were used in dozens of historic experiments to try to find out what cathode rays were. There were two theories: British scientists Crookes and Cromwell Varley believed they were 'corpuscles' or 'radiant matter', that is, electrically charged atoms. German researchers E. Wiedemann, Heinrich Hertz, and
Eugen GoldsteinEugen Goldstein was a German physicist. He was an early investigator of discharge tubes, the discoverer of anode rays, and is sometimes credited with the discovery of the proton.- Life :...
believed they were '
aetherIn the late 19th century, luminiferous aether or ether, meaning light-bearing aether, was the term used to describe a medium for the propagation of light....
vibrations', some new form of electromagnetic waves, and were separate from what carried the current through the tube. The debate continued until
J. J. ThomsonSir Joseph John "J. J." Thomson, OM, FRS was a British physicist and Nobel laureate. He is credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer...
measured their mass, proving they were a previously unknown negatively charged particle, which he called a 'corpuscle' but was later named
electron.
Maltese cross
Juliusz Plücker in 1869 built an anode shaped like a
Maltese CrossThe Maltese cross, also known as the Amalfi cross, is identified as the symbol of an order of Christian warriors known as the Knights Hospitaller or Knights of Malta and through them came to be identified with the Mediterranean island of Malta and is one of the National symbols of Malta...
in the tube. It was hinged, so it could fold down against the floor of the tube. When the tube was turned on, it cast a sharp cross-shaped shadow on the fluorescence on the back face of the tube, showing that the rays moved in straight lines. After a while the fluorescence would get 'tired' and decrease. If the cross was folded down out of the path of the rays, it no longer cast a shadow, and the previously shadowed area would fluoresce stronger than the area around it.
Perpendicular emission
Eugen GoldsteinEugen Goldstein was a German physicist. He was an early investigator of discharge tubes, the discoverer of anode rays, and is sometimes credited with the discovery of the proton.- Life :...
in 1876 found that cathode rays were always emitted perpendicular to the cathode's surface. If the cathode was a flat plate, the rays were shot out in straight lines perpendicular to the plane of the plate. This was evidence that they were particles, because a luminous object, like a red hot metal plate, emits light in all directions, while a charged particle will be repelled by the cathode in a perpendicular direction. If the electrode was made in the form of a concave spherical dish, the cathode rays would be focused to a spot in front of the dish. This could be used to heat samples to a high heat.
Deflection by electric fields
Heinrich Hertz built a tube with a second pair of metal plates to either side of the cathode ray beam, a crude
CRTThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
. If the cathode rays were
charged particleIn physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...
s, their path should be bent by the
electric fieldIn physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
created when a
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
was applied to the plates, causing the spot of light where the rays hit to move sideways. He didn't find any bending, but it was later determined that his tube was insufficiently evacuated, causing accumulations of
surface chargeSurface charge is the electric charge present at an interface. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field...
which masked the electric field. Later Artur Shuster repeated the experiment with a higher vacuum. He found that the rays were attracted toward a positively charged plate and repelled by a negative one, bending the beam. This was evidence they were negatively charged, and therefore not electromagnetic waves.
Deflection by magnetic fields
Crookes put a
magnetA magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
across the neck of the tube, so that the North pole was on one side of the beam and the South pole was on the other, and the beam travelled through the
magnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
between them. The beam was bent down, perpendicular to the magnetic field. This was similar to the behavior of electric currents in an electric generator and showed that the cathode rays obeyed
Faraday's lawFaraday's law may refer to the following:*Faraday's laws of electrolysis in chemistry*Faraday's law of induction, also known as Faraday-Lenz Law, in electromagnetism physics**The Maxwell–Faraday equation...
like currents in wires.
Paddlewheel
Crookes put a tiny vaned
turbineA turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
or paddlewheel in the path of the cathode rays, and found that it rotated when the rays hit it. The paddlewheel turned in a direction away from the cathode side of the tube, suggesting that the rays were coming from the cathode. Crookes concluded at the time that this showed that cathode rays had
momentumIn classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
, so the rays were likely
matterMatter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
particles. However later it was proposed that the paddle wheel turned not due to the momentum of the particles (or electrons) hitting the paddle wheel but due to the
radiometric effectThe Crookes radiometer, also known as the light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement...
. When the rays hit the paddle surface they heated it, and the heat caused the gas next to it to expand, pushing the paddle. This was proven in 1903 by J. J. Thompson who calculated that the momentum of the electrons hitting the paddle wheel would only be sufficient to turn the wheel one revolution per minute. So all this experiment really demonstrated was the ability of cathode rays to heat objects.
Charge
Jean-Baptiste Perrin wanted to determine whether the cathode rays actually carried negative
chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
, or whether they just accompanied the charge carriers, as the Germans thought. In 1895 he constructed a tube with a 'catcher', a closed aluminum cylinder with a small hole in the end facing the cathode, to collect the cathode rays. The catcher was attached to an
electroscopeAn electroscope is an early scientific instrument that is used to detect the presence and magnitude of electric charge on a body. It was the first electrical measuring instrument. The first electroscope, a pivoted needle called the versorium, was invented by British physician William Gilbert...
to measure its charge. The electroscope showed a negative charge, proving that cathode rays really carry negative electricity.
Canal rays
Goldstein found in 1886 that if the cathode is made with small holes in it, streams of a faint luminous glow will be seen issuing from the holes on the back side of the cathode, facing away from the anode. It was found that in an electric field these
anode rayAnode rays are beams of positive ions that are created by certain types of gas discharge tubes. They were first observed in Crookes tubes during experiments by the German scientist Eugen Goldstein, in 1886. Later work on anode rays by Wilhelm Wien and J. J...
s bend in the opposite direction from cathode rays, toward a negatively charged plate. These were the positive
ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s which were attracted to the cathode, and created the cathode rays. They were named
canal rays (
Kanalstrahlen) by Goldstein.
Doppler shift
Eugen GoldsteinEugen Goldstein was a German physicist. He was an early investigator of discharge tubes, the discoverer of anode rays, and is sometimes credited with the discovery of the proton.- Life :...
thought he had figured out a method of measuring the speed of cathode rays. If the glow discharge seen in the gas of Crookes tubes was produced by the moving cathode rays, the light radiated from them in the direction they were moving, down the tube, would be shifted in
frequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
due to the
Doppler effectThe Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
. This could be detected with a spectroscope because the emission line
spectrumA spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
would be shifted. He built a tube shaped like an "L", with a spectroscope pointed through the glass of the elbow down one of the arms. He measured the spectrum of the glow when the spectroscope was pointed toward the cathode end, then switched the power supply connections so the cathode became the anode and the electrons were moving in the other direction, and again observed the spectrum looking for a shift. He did not find one, which he calculated meant that the rays were traveling very slowly. It is now recognized that the glow in Crookes tubes is emitted from gas atoms hit by the electrons, not the electrons themselves. Since the atoms are thousands of times more massive than the electrons, they move much slower, accounting for the lack of Doppler shift.
Lenard window
Philipp LenardPhilipp Eduard Anton von Lenard , known in Hungarian as Lénárd Fülöp Eduárd Antal, was a Hungarian - German physicist and the winner of the Nobel Prize for Physics in 1905 for his research on cathode rays and the discovery of many of their properties...
wanted to see if cathode rays could pass out of the Crookes tube into the air. He built a tube with a "window" in the glass envelope made of aluminum foil just thick enough to hold the atmospheric pressure out (later called a
Lenard window) facing the cathode so the cathode rays would hit it. He found that something did come through. Holding a fluorescent screen up to the window caused it to fluoresce, even though no light reached it. A
photographic platePhotographic plates preceded photographic film as a means of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...
held up to it would be darkened, even though it wasn't exposed to light. The effect had a very short range of about 2.5 centimeters. He measured the ability of cathode rays to penetrate sheets of material, and found they could penetrate much farther than moving atoms could. Since atoms were the smallest particles known at the time, this was first taken as evidence that cathode rays were waves. Later it was realized that electrons were much smaller than atoms, accounting for their greater penetration ability. Lenard was awarded the
Nobel Prize in PhysicsThe Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
for 1905 for his work.
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