K10 Passage of electricity through gases
We understand by space charge the charge of the ions and electrons in a space, which as mobile charge carriers form the convection current (flowing through the space) and transport the electricity through the space - the negative ones to the anode, the positive ones to the cathode. The convection current can be compared with a current of fluid, which displaces like a cylinder, uniformly filled with charge carriers ( electrons, ions). For example, if electrons move at the same velocity v linearly and along parallels through space and bypass each second n electrons through a unit area, perpendicular to their direction of motion,the cylinder moves forward in one second by v; it contains n electrons with the total charge ve. The density r of the space charge is the charge within unit volume, that is ne/v. However, ve is equal to the amount of charge passing in unit time through unit area, that is, it is equal to the current density j of the convection current, whence r = j/v. These considerations only concern the charge carriers moving in one direction. If n- flows with the charge e- and velocity v- in one direction, n+ with the charge e+ and velocity v+ moves in the opposite direction, the density of the total convection current is j = j++ j- = n+e+ + n-e- , but the density of the space charge is r=r++ r- = n+e+/v+ - n-e-/n- = j+/v+ - j-/v-. In fact, the space charge is composed algebraically of the partial charges, the total current by addition of the absolute values of the partial currents. Hence the space charge density can vanish, although a strong convection current flows, in fact, if the partial current densities are interrelated as the stream velocities of the corresponding carriers.
Properties and nature of cathode rays
The internal natures of cathode and light rays differ fundamentally, even though some of the properties of the former recall those of the latter.
1. Cathode rays cause many substances to become fluorescent and phosphorescent: A ruby radiates brilliantly red, zinc sulphide and sulphur calcium bluish green, the glass wall of a discharge tube fluorescent light green, where the cathode rays meet them (it was just this which led to the discovery of these rays and simplified their study).
2. Cathode rays propagate linearly. A metal - the cross in Fig. 515 - makes a shadow, because it keeps the cathode rays away from the wall (4. below); at other locations, the wall fluoresces at their impact.
3. Cathode rays emit perpendicularly to the surface of the anode. If it is formed like a hollow mirror, the rays converge in a focal point. A phosphorescent substance radiates there with most light.
4. Metal foils, 1/1000 to 1/100 mm thick, which do not let light pass, allow fast cathode rays to pass; however, thicker layers of whatever material - also the glass wall of the tube - do not. Philipp Eduard Anton Lenard 1862-1947 1894 has replaced a spot of the glass wall opposite the cathode by aluminium foil and conducted the rays through this aluminium window out of the tube (Fig. 516). Coolidge 1926 developed this idea into a window with a diameter of 80 mm and conducted the rays through the aluminium window of an 1½ m long tube with enormous actions of the rays at tensions of 350000 Volt.
5. If cathode rays penetrate any substances - for example, air - they are dispersed diffusely, similarly to rays of light in milky water. If they travel while escaping from the aluminium window along a surface, covered with phosphorescent material, a cluster is formed which widens with the distance from the window (Fig. 517, the dotted lines show the form of the bundle of rays, to be expected during undisturbed linear propagation).
6. On absorption, cathode rays generate considerable heat. Lenard's window, which, in fact, absorbs part of the radiation, may then heat up and melt. At the focal point of a hollow mirror cathode, you can heat up metals, which are hard to melt, to white heat and evaporate them.
7. Magnetic and electric fields deflect cathode rays. For example, if you place a magnet near the instrument in Fig. 515, the shadow of the cross will move. A positively charged plate attracts them, a negatively charged plate repels them, that is, cathode rays carry a negative charge. Measurements of the electric and magnetic ability of cathode rays to be deflected divulge their nature.
Deflection of cathode rays in a magnetic field (Hittorf 1869)
Cathode rays consist of fast moving, electrically charged particles (electrons), which simulate rays by their very large velocity; like every other electric current, they generate a magnetic field. In contrast, an external magnetic field forces the particles to move along circular or screw-shaped tracks, just as a magnet attempts to force a flexible conductor into such shapes. However, cathode rays display this phenomenon much more clearly, since here the carriers of electricity are on their own without a material conductor the stiffness of which they must overcome.
The force acting between a magnetic field and a cathode ray is readily computed, if you assume the magnetic field to be constant throughout and its lines of force to be at right angle to the direction of the cathode rays. If the field has the strength H, then the force, which it exerts on a cathode ray particle e moving with the velocity v, is according to the fundamental electro-dynamics law H·e·v. This force acts perpendicularly to the lines of force of the field and to the direction of motion of the particle, so that it is deflected from its straight path into a curved one, which is everywhere perpendicular to the field of force. Due to the curvature of its track, the particle experiences a centrifugal acceleration v²/r and with it a force m·v²/r, where m is the mass of the particle and r the radius of curvature of its track at that location. The particle will follow a track, along which the magnetic force and the centrifugal force are in equilibrium, that is,
Hev = mv²/r or He = mv/r. (1)
To start with, this equation is only valid for a small element of the trajectory. However, not only H, e and m are constant, but also the velocity v does not change, because the accelerating forces, acting on the particle, neutralize each other. Hence also the radius of curvature r is constant, that is, the particle moves around a circle with radius r in a homogeneous field of force, which acts at right angle to its direction of motion (Fig. 518).The smaller its velocity, the smaller is the radius of the circle. If the cathode ray enters a magnetic field at an angle, the trajectory becomes screw-formed.
Deflection of cathode rays in an electric field (Goldstein 1876 Hertz 1883)
Fig. 519 shows a discharge tube, which has in addition to the cathode K and the anode A, with a fine opening, two parallel, interconnected metal plates F1 and F2, one of which is connected to the -pole, the other to the +pole of an accumulator. To start with, let the cathode ray move parallel to the plates (from K to A), that is, perpendicularly to the lines of force of the field. The field accelerates the particle constantly in the direction of the lines of force, that is, like the vertically downwards directed force of gravity acts on a horizontally projected body. Assuming the ray to be moving in a space without a field (that is, when the plates are earthed) exactly in the central plane between the two plates: By what distance s is the ray as it leaves the field displaced with respect to the initial direction, if the field has the strength F and the length a? If instead of the electric field, there were present the field of gravity, then you would have s = gt²/2, where t is the time during which the gravitational field acts on the body, that is, in this case the time interval between the entry and exit from the electric field. If the ray passes the a cm long field at the velocity v, then t = a/v. moreover, the force, which acts in the electric field on the cathode ray particle, is given by the equation mg = eF. Hence there acts in the electric field the acceleration g = eF/m. Substituting for t and g into the initial equation, we find
s = (eFa²)/2(mv²) (2).
Note that in the electric field, in contrast to the magnetic field, the velocity of the cathode rays changes all the time. Of course, during the motion of a ray as in Fig. 519, the difference between the velocities at the entry into and exit from the field is very small.
You employ electric deflection of cathode rays in the tube, named after Braun, for the measurement of rapidly changing Voltages (Fig. 519). The anode A with a hole separates out of the rays from the cathode K a fine bundle, which running between the plates F1F2 eventually meets the fluorescent screen S and causes there a bright spot of light. For example, if you discharge a Leyden jar, the covers of which are connected to F1 and F2, the concurrent electric oscillations cause corresponding oscillations of the field F1F2, which are followed by the cathode rays, whence the spot of light on the screen oscillates at the same frequency as the discharge of the jar. You observe the spot of light in a rotating mirror, in which you see the rapid vibrations side by side (Fig. 520, oscillograph).
Consequences of the magnetic and electric property of deflection of cathode rays
You keep the gas pressure and discharge tension in a cathode ray tube constant. At first, let a magnetic field of strength H act on the bundle of cathode rays, let the bundle describe a circular track with radius r; then guide the bundle into an electric field of strength F, as a result of which there occurs the deflection s. Substitute the values found into the equations (1) and (2) and measure all quantities in the same measuring system, for example, in electromagnetic CGS-units. The two equations contain the three unknowns: Velocity, charge and mass of a particle. Since we do not have a third equation, we will consider the velocity as one unknown, the ratio of the charge to the mass (e/m) - the specific charge - as the second unknown. We can do this, because e and m enter these equations only in this ratio. Whatever are our experiments, the chosen electrodes and gas pressure: We will always find for the specific charge e/m of the cathode ray particle: 1.76·10 electromagnetic units* (cf. below).
A comparison of this number with the specific charge of the hydrogen ion yields new insight of fundamental importance. An amount of electricity of 95730 Coulomb (equal to 9573 electro-magnetic units in the CGS-system) must pass through an electrolyte, in order to separate 1 g hydrogen. This quantity of electricity must be completely attached to hydrogen atoms, that is, to hydrogen ions, since otherwise the quantities, separated by the current, could not be proportional to the atomic weights, as Faraday's law requires. Hence the ratio between the total quantity of electricity E, passing through the electrolyte, and the the total separated amount of substance (1 g H2) is just as large as the ratio of the charge of a single H-ion eH to the mass mH of this ion, that is, E/1 = eH/mH = 9573 electromagnetic units. Thus, for the hydrogen atom, the ratio e/m is 1839 times smaller as (cf. above) the corresponding ratio for the cathode ray particle. However, the charge of an H-ion is as large as that of a single cathode ray particle. Hence the difference can only be due to the mass. The cathode ray particle, the mass of which therefore must be 1839 times smaller than that of the hydrogen atom - this also contradicts the view that atoms are the smallest components of matter - cannot consist of the atom of a known element; it must be a hitherto unknown, almost mass free object, indeed, an object of a universal kind, for these particles are always of the same kind whatever is the gas in the discharge tube or the material of the electrodes. It is not the mass, but the electric charge which determines the character of these particles; they are atoms of negative electricity. Following a suggestion by Stoney 1891, they are called electrons. An independent, positive electricity atom has hitherto only been found under special experimental conditions. As a rule, positive electricity is tied to mass. The smallest is H+ - the proton.
Electrons with a different origin
Electrons do not only arise in the form of cathode rays, but also during many other physical (and also chemical) processes. Electrons emit from many substances, as they are met by short wave light, especially metals and metal alloys (light-electric electron emission or, more briefly, photo-effect). The spark distance of an inductor becomes larger as you irradiate the negative pole with ultraviolet light. This observation let to the discovery by Hallwachs 1888 that a pure zinc plate changes itself positive as it is irradiated by an arc lamp; if the plate has beforehand been charged negatively, it loses its charge during irradiation. You demonstrate the fact that this phenomenon has been provoked by the ultraviolet part of the light by placing a glass plate between the plate and the arc light. The glass plate absorbs the ultraviolet light and the charging stops.
Fig. 521 shows a simple arrangement for the measurement of the photo-effect. Ultraviolet light enters through the quartz window F, which is especially permeable for these rays, into a highly evacuated glass tube G to a plate A made of zinc. The electrometer, connected to A, soon displays a positive charge, whence the negative electricity exits from there. Lenard has shown that one is here concerned with electrons by separating by means of the hole in the counter plate B a narrow bundle of rays. This bundle meets the plate a, which collects the charge travelling with the bundle and thereby indicates the existence of the radiation by an electrometer connected to a. If you bring a magnet close to the tube in a suitable manner, you find the charge on the plate b instead of on the plate a, a sign that the invisible ray was really deflected by the magnet, and indeed in a sense corresponding to cathode rays. The quantitative execution of the experiment yields for e/m the same value as for cathode rays, a proof that they are electrons, released from A by the ultraviolet light. Simultaneously, we learn that the zinc atoms contain electrons, which are so weakly attached to the atoms that the incident light separates them. We will show that electrons are fundamental components of all atoms .
experiment, first described, the positive charging of the plate A
ends as soon as the electrometer, attached to A,
indicates a few Volts. In fact, the starting velocities of the
electrons are so small that a charge of a few Volts is sufficient
to draw them back to the irradiated plate. In order to obtain fast
rays, we need only place on the plate B a strong
positive charge. Then the electrons are accelerated in the
electric field between A and B. By changing the
strength of this field, we can regulate arbitrarily the velocity
of the rays. - Two very remarkable laws have been determined:
1. The velocity of the emitted electrons does not depend on the intensity of the agitating light; it is the larger, the shorter is its wave length. For example, X-rays with their extremely short waves release photo-electrons, the velocity of which is as large as that of the cathode rays in an discharge tube with high tension. The relationship between the exciting wave length and the velocity of the electrons can be strictly formulated mathematically in a law, which is of greatest importance for the development of quantum theory.
2. The number of electrons, released photo-electrically, is exactly proportional to the strength of the exciting light. The strict validity of this law has led to the construction of photo-cells, which in connection with electrometers yield very sensitive photo-meters - for example, for use in star photometry (Julius Elster 1854-1920, Hans Friedrich Geitel 1855-1923). A photo-cell (light electric cell) consists of a glass vessel G (Fig. 522) with a diameter of a few centimetres; its internal surface is covered partly with potassium, which is also light-electric sensitive in the visible spectrum. A first platinum wire, passing air tight through the glass, connects the potassium surface to the negative pole of an accumulator, a second one P', leading far into the sphere and serving as an anode, is connected to the electrometer. The light, the intensity of which is to be measured, enters through the window G on to the potassium surface K, which serves as cathode, and detaches there electrons. By the action of the electric field between K and P', these electrons are driven to the wire P' so that the electrometer gets charged. This charge is exactly proportional to the intensity of the light. In order to increase the sensitivity of the cell, you increase the charge, flowing to the electrometer, by the earlier discussed principle of collision ionization. For this purpose, the cells are mostly filled with argon.
Emission of electrons at high temperatures
It has been known for a long time that flames conduct electricity and glowing bodies readily part with electric charge. The theory of electrons explains satisfactorily these and other related phenomena. As they glow, metals, coal and above all the oxides of the alkaline earths (barium, strontium, calcium) emit electrons in large numbers; however, their velocity is very small. If you employ as cathode a platinum wire (Wehnelt), covered with such an oxide, and cause it to glow (by a battery of a few accumulators), you can already with a discharge tension of 100 - 200 Volt, that is, with a common domestic source of electricity, send current of several Amperes through the discharge tube. This becomes possible by the fact that due to the high electron emission of the glowing wire the cathode-drop more or less disappears. If the cathode is cold, the electrons, required for the passage of the current, must first be generated by the strong electric field near the cathode (cathode-drop). Hence the current only starts at a much higher tension and also attains only an intensity of a few milli-Amps. If the wire glows only weakly, so that due to the reduced electron emission a smaller cathode-drop develops, there arise cathode rays of great intensity, but small velocity, which are readily diverted by electric and magnetic fields.
The emission of electrons from glowing substances can be compared with evaporation (Richardson). Between the emitting and remaining electrons develops a static equilibrium like between the steam of water and evaporating water.
The X-ray tube of Coolidge is based on the electron emission of glowing wires. The electrons, required for its operation, come exclusively from a glowing wolfram wire, the glowing of which is maintained by its own circuit (accumulator). All parts of the tube have the best possible vacuum as well as the tube itself, so that also at the highest tension no current passes between the cold wolfram wire and the anode. As the number of electrons rises with the temperature, you can vary the emission of electrons arbitrarily and thereby generate any degree of hardness, that is, you can regulate at a given tension the hardness of the rays by the temperature of the glowing wire.
Amplifier tube (Robert von Lieben 1906)
An important application of the electron emission of glowing metals are the amplifier tubes (electron tubes, glowing cathode tubes). These tubes (which have already been referred to as electric valves) serve to amplify substantially electric currents, which are so weak that even sensitive measuring devices cannot indicate them. In Fig. 524, the very well evacuated glass tube E has four wires leading into it; the first goes to a disk formed anode A, the second to a wire net (lattice) G, the last two to a cathode K, the part of which near the lattice consists of a thin wire, which can be caused to glow by means of the accumulator B1. You now connect the anode A to the +pole and the (at first cold) cathode K to the -pole of the battery B of about 50 Volt. The cathode K is simultaneously earthed, so that a positive tension of 50 Volt is at A. The sensitive galvanometer F in the circuit does not move, since a tension of 50 Volt is insufficient for driving a discharge through a well evacuated tube. However, if we gradually cause the cathode to glow, we soon observe a movement of the galvanometer which becomes stronger as the temperature of the cathode increases.
How does this current arise? We know already that an electric current through a gas filled or also gas free tube is only possible if there exist electric charges as carriers of this current. A glowing wire and with it also our cathode supply such charges, that is, electrons in large numbers. The electrons are grasped by the electric field, which the battery B2 maintains between A and K, and wander more or less following the lines of force from the cathode through the lattice to the anode. This displacement of electrons takes negative charge from the cathode to the anode, whence: An electric current flows through the tube. This current, which the galvanometer F can measure, is called anode current.
We will now explain the purpose of the lattice the electric tension of which we have not mentioned hitherto. For example, if we give the wire net a negative tension of 10 Volt, the electrons, which previously went to the anode, are driven back to the cathode by the repelling action of the lattice; hence no more electrons pass through the lattice and the anode current vanishes. On the other hand, if we give the lattice a positive tension of 10 Volt, the lattice supports the anode in its action on the electrons and the anode current is amplified. Fig. 525 demonstrates in detail the changes in the anode current, when we allot to the lattice in a definite tube different tensions. The abscissae are the tensions on the lattice - to the right of the origin positive, to the left of it negative. The ordinates are the values of the anode current, belonging to the associated lattice tension, read off at the galvanometer F. We see now, in agreement with the preceding considerations, that for larger negative lattice tensions the anode current vanishes completely and that, on the other hand, it attains for higher tensions a maximum value (which also does not rise further as the lattice tension is raised, because the glowing cathode can only supply at a given temperature a certain number of electrons per second). Between the extreme values of the anode current lies a range in which even a minute change of the tension of the lattice causes a considerable change of the anode current: If we change the tension of the lattice from - 1 Volt to + 1 Volt, the anode current rises from 5.5 to 9.5 milli-Amp! This dependence of the anode current on the tension of the lattice can be employed to amplify a current.
Apply between two connection clamps alternating tension at 50 periods. In order to confirm the presence of this alternating tension, it suggests itself to connect a telephone to the two clamps. If the alternating tension can deliver an appreciable current through the telephone, it will respond and give a tone characteristic for the frequency 50. But it can also happen - this occurs frequently during wireless reception of electric waves - that, while the alternating tension is present, it drops immediately to an undetctable value, when you want to operate a measuring device, for example, a telephone. It is at this stage that the amplifier tube provides help. We connect the two pole clamps to the cathode and lattice and thus cause the lattice tension to oscillate 100 times per second between two extreme values of the tension, say, -1 and + 1 Volt. This alternating tension of the lattice now steers the anode current in the same rhythm; every time when the lattice tension becomes negative, the anode current drops and it grows again when the lattice tension reaches positive values. A telephone, replacing in the anode circuit the galvanometer, responds immediately, since now an alternating current of sufficient strength flows through the circuit. This current is delivered by the battery B2 and no more by the alternating tension between the pole clamps. We only demand of this alternating tension that it also transfers itself to the lattice. The energy required for this is minute and would not by far be sufficient to operate the telephone directly. We can do this only by introduction of the amplifier tube, which steered by the alternating tension supplies the current required by the telephone.
Amplifier tubes are built depending on their special purpose in various designs of the internal electrodes. Depending on a tube's design, its characteristic changes, that is, the curve which represents the anode current in dependence on the lattice tension. Amplifier tubes are also employed in engineering in a different manner from that described here, especially in wireless telegraphy for the generation of undamped electric waves of arbitrary frequency.
If a current passes through a discharge tube, the cathode of which has a hole (Fig. 526), a ray bundle S emits out of the hole at a gas pressure of about 0.05 mm mercury in the opposite direction of the cathode rays; its colour is yellowish in air, pink in hydrogen. Following Goldstein 1886, who discovered these rays using a cathode with many canals, they are referred to as canal rays. In an electric or magnetic field, the canal rays are diverted oppositely to the cathode rays, that is, they carry a positive charge. Moreover: Cathode rays are already visibly deflected in the proximity of a simple horseshoe magnet, deflection of canal rays can only be achieved by much stronger fields (electro-magnet). The reason is: Canal rays consist of electrically charged atoms of the gas in the discharge tube, that is, they have in contrast to cathode rays considerable mass.
How do canal rays arise? A neutral gas atom loses by the discharge process one electron. As positive ion, it now follows the lines of electric force and is accelerated in the cathode-fall in the opposite direction to the cathode rays. Eventually, it hits the cathode and is absorbed there unless it cannot emit through one of the holes as canal ray at the back of the cathode. Hence, although the canal ray is accelerated by the same electric field as the cathode ray, its velocity is nevertheless considerably smaller. While the charge of the particle is both times the same, whence the accelerating force is the same, the mass to be accelerated in the canal ray is large, in the cathode ray small.
You speak of hydrogen-, helium- etc. canal rays, depending on the kind of atom constituting the rays. Apart from the atom rays, there occur under certain circumstances also molecule rays, for example, H2, O2, CO, CO2 rays, which can carry simple or also multiple charges, corresponding to a loss of one or several electrons. Strange to say, there exist also canal rays without or negative charge; however, these are secondary phenomena which must be due to collisions of the canal gas particles with gas molecules. In fact, during such collisions, the canal ray particles can take in split off electrons and thus neutralize their charge and reverse its sign. If the gas pressure in a tube is not extremely small, a canal ray particle changes very often during its flight its charge in this manner (charge shift).
If you direct a fine bundle of canal rays with particles of different mass, but equal charge and velocity, into a magnetic or electric field, it is spread out like a fan, because the lightest atoms are diverted furthest, the heaviest ones least from their initial track. You can determine atomic weights in this manner. A difficulty is raised by the alternating charge of the particles and the non-uniform velocity, caused by it. J.J. Thomson and Wien opened up new approaches and created the foundations for work by Aston, which has guided our concepts regarding the constitution of chemical elements in new directions.
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